diff --git a/libs/base/sim/control.ts b/libs/base/sim/control.ts
index 849c3ac65..70048c713 100644
--- a/libs/base/sim/control.ts
+++ b/libs/base/sim/control.ts
@@ -20,6 +20,7 @@ namespace pxsim.BufferMethods {
     }
 
     export function hash(buf: RefBuffer, bits: number) {
+        BufferMethods.typeCheck(buf);
         bits |= 0
         if (bits < 1)
             return 0
diff --git a/libs/cable/sim/cable.ts b/libs/cable/sim/cable.ts
index ee3bebdde..d0ad15860 100644
--- a/libs/cable/sim/cable.ts
+++ b/libs/cable/sim/cable.ts
@@ -1,5 +1,6 @@
 namespace pxsim.network {
     export function cableSendPacket(buf: RefBuffer): void {
+        BufferMethods.typeCheck(buf);
         const state = getCableState();
         state.send(buf);
     }
@@ -12,7 +13,7 @@ namespace pxsim.network {
     export function onCablePacket(body: RefAction): void {
         const state = getCableState();
         state.listen(body);
-    }    
+    }
 
     export function onCableError(body: RefAction): void {
         const state = getCableState();
diff --git a/libs/core/sim/neopixel.ts b/libs/core/sim/neopixel.ts
index 2636673d0..5ca087318 100644
--- a/libs/core/sim/neopixel.ts
+++ b/libs/core/sim/neopixel.ts
@@ -49,6 +49,7 @@ namespace pxsim {
     }
 
     export function sendBufferAsm(buffer: RefBuffer, pin: number) {
+        BufferMethods.typeCheck(buffer);
         const b = board();
         if (!b) return;
         const p = b.edgeConnectorState.getPin(pin);
@@ -63,6 +64,7 @@ namespace pxsim {
 namespace pxsim.light {
     // Currently only modifies the builtin pixels
     export function sendBuffer(pin: { id: number }, clk: { id: number }, mode: number, b: RefBuffer) {
+        BufferMethods.typeCheck(b);
         const state = neopixelState(pin.id);
         if (!state) return;
         state.mode = mode & 0xff;
diff --git a/libs/core/sim/pins.ts b/libs/core/sim/pins.ts
index 89d114c72..778b04e82 100644
--- a/libs/core/sim/pins.ts
+++ b/libs/core/sim/pins.ts
@@ -152,7 +152,7 @@ namespace pxsim.pins {
         markUsed(scl);
         return b && b.edgeConnectorState && b.edgeConnectorState.createI2C(sda, scl);
     }
-   
+
     export function createSPI(mosi: DigitalInOutPin, miso: DigitalInOutPin, sck: DigitalInOutPin) {
         const b = board() as EdgeConnectorBoard;
         markUsed(mosi);
@@ -168,6 +168,7 @@ namespace pxsim.I2CMethods {
     }
 
     export function writeBuffer(i2c: I2C, address: number, buf: RefBuffer, repeat?: boolean): number {
+        BufferMethods.typeCheck(buf);
         return 0;
     }
 }
@@ -179,6 +180,8 @@ namespace pxsim.SPIMethods {
     }
 
     export function transfer(device: pxsim.SPI, command: RefBuffer, response: RefBuffer) {
+        BufferMethods.typeCheck(command);
+        BufferMethods.typeCheck(response);
         device.transfer(command, response);
     }
 
diff --git a/libs/core/sim/serial.ts b/libs/core/sim/serial.ts
index 4488c7d1b..e4390b371 100644
--- a/libs/core/sim/serial.ts
+++ b/libs/core/sim/serial.ts
@@ -16,6 +16,7 @@ namespace pxsim.SerialDeviceMethods {
     }
 
     export function writeBuffer(device: SerialDevice, buffer: RefBuffer) {
+        BufferMethods.typeCheck(buffer);
         device.writeBuffer(buffer);
     }
 
diff --git a/libs/infrared/sim/ir.ts b/libs/infrared/sim/ir.ts
index daaa3451c..bbaf1c00c 100644
--- a/libs/infrared/sim/ir.ts
+++ b/libs/infrared/sim/ir.ts
@@ -1,5 +1,6 @@
 namespace pxsim.network {
     export function infraredSendPacket(buf: RefBuffer): void {
+        BufferMethods.typeCheck(buf);
         const state = getInfraredState();
         state.send(buf);
     }
@@ -12,7 +13,7 @@ namespace pxsim.network {
     export function onInfraredPacket(body: RefAction): void {
         const state = getInfraredState();
         state.listen(body);
-    }    
+    }
 
     export function onInfraredError(body: RefAction): void {
         const state = getInfraredState();
diff --git a/libs/music/sim/music.ts b/libs/music/sim/music.ts
index 457a92c09..eacec432e 100644
--- a/libs/music/sim/music.ts
+++ b/libs/music/sim/music.ts
@@ -43,6 +43,7 @@ namespace pxsim.music {
     }
 
     export function setTone(buffer: RefBuffer) {
+        BufferMethods.typeCheck(buffer);
         // TODO: implement set tone in the audio context
     }
 
diff --git a/libs/radio/sim/radio.ts b/libs/radio/sim/radio.ts
index e9ff84731..9429633de 100644
--- a/libs/radio/sim/radio.ts
+++ b/libs/radio/sim/radio.ts
@@ -14,12 +14,13 @@ namespace pxsim.radio {
         state.setTransmitPower(power);
     }
 
-    export function setFrequencyBand(band: number) { 
+    export function setFrequencyBand(band: number) {
         const state = pxsim.getRadioState();
         state.setFrequencyBand(band);
     }
 
     export function sendRawPacket(buf: RefBuffer) {
+        BufferMethods.typeCheck(buf);
         let cb = getResume();
         const state = pxsim.getRadioState();
         if (state.enable) {
diff --git a/libs/screen/sim/image.ts b/libs/screen/sim/image.ts
index 561dbb395..5d2a6ea2d 100644
--- a/libs/screen/sim/image.ts
+++ b/libs/screen/sim/image.ts
@@ -64,34 +64,53 @@ namespace pxsim.ImageMethods {
     export function XX(x: number) { return (x << 16) >> 16 }
     export function YY(x: number) { return x >> 16 }
 
-    export function width(img: RefImage) { return img._width }
+    export function width(img: RefImage) {
+        typeCheck(img);
+        return img._width;
+    }
 
-    export function height(img: RefImage) { return img._height }
+    export function height(img: RefImage) {
+        typeCheck(img);
+        return img._height;
+    }
 
-    export function isMono(img: RefImage) { return img._bpp == 1 }
+    export function isMono(img: RefImage) {
+        typeCheck(img);
+        return img._bpp == 1
+    }
 
-    export function isStatic(img: RefImage) { return img.gcIsStatic() }
+    export function isStatic(img: RefImage) {
+        typeCheck(img);
+        return img.gcIsStatic()
+    }
 
-    export function revision(img: RefImage) { return img.revision }
+    export function revision(img: RefImage) {
+        typeCheck(img);
+        return img.revision
+    }
 
     export function setPixel(img: RefImage, x: number, y: number, c: number) {
+        typeCheck(img);
         img.makeWritable()
         if (img.inRange(x, y))
             img.data[img.pix(x, y)] = img.color(c)
     }
 
     export function getPixel(img: RefImage, x: number, y: number) {
+        typeCheck(img);
         if (img.inRange(x, y))
             return img.data[img.pix(x, y)]
         return 0
     }
 
     export function fill(img: RefImage, c: number) {
+        typeCheck(img);
         img.makeWritable()
         img.data.fill(img.color(c))
     }
 
     export function fillRect(img: RefImage, x: number, y: number, w: number, h: number, c: number) {
+        typeCheck(img);
         if (w == 0 || h == 0 || x >= img._width || y >= img._height || x + w - 1 < 0 || y + h - 1 < 0)
             return;
         img.makeWritable()
@@ -114,6 +133,8 @@ namespace pxsim.ImageMethods {
     }
 
     export function mapRect(img: RefImage, x: number, y: number, w: number, h: number, c: RefBuffer) {
+        typeCheck(img);
+        BufferMethods.typeCheck(c);
         if (c.data.length < 16)
             return
         img.makeWritable()
@@ -138,6 +159,7 @@ namespace pxsim.ImageMethods {
     }
 
     export function equals(img: RefImage, other: RefImage) {
+        typeCheck(img);
         if (!other || img._bpp != other._bpp || img._width != other._width || img._height != other._height) {
             return false;
         }
@@ -153,6 +175,8 @@ namespace pxsim.ImageMethods {
     }
 
     export function getRows(img: RefImage, x: number, dst: RefBuffer) {
+        typeCheck(img);
+        BufferMethods.typeCheck(dst);
         x |= 0
         if (!img.inRange(x, 0))
             return
@@ -172,6 +196,8 @@ namespace pxsim.ImageMethods {
     }
 
     export function setRows(img: RefImage, x: number, src: RefBuffer) {
+        typeCheck(img);
+        BufferMethods.typeCheck(src);
         x |= 0
         if (!img.inRange(x, 0))
             return
@@ -191,12 +217,14 @@ namespace pxsim.ImageMethods {
     }
 
     export function clone(img: RefImage) {
+        typeCheck(img);
         let r = new RefImage(img._width, img._height, img._bpp)
         r.data.set(img.data)
         return r
     }
 
     export function flipX(img: RefImage) {
+        typeCheck(img);
         img.makeWritable()
         const w = img._width
         const h = img._height
@@ -207,6 +235,7 @@ namespace pxsim.ImageMethods {
 
 
     export function flipY(img: RefImage) {
+        typeCheck(img);
         img.makeWritable()
         const w = img._width
         const h = img._height
@@ -225,6 +254,7 @@ namespace pxsim.ImageMethods {
     }
 
     export function transposed(img: RefImage) {
+        typeCheck(img);
         const w = img._width
         const h = img._height
         const d = img.data
@@ -244,6 +274,8 @@ namespace pxsim.ImageMethods {
     }
 
     export function copyFrom(img: RefImage, from: RefImage) {
+        typeCheck(img);
+        typeCheck(from);
         if (img._width != from._width || img._height != from._height ||
             img._bpp != from._bpp)
             return;
@@ -251,6 +283,7 @@ namespace pxsim.ImageMethods {
     }
 
     export function scroll(img: RefImage, dx: number, dy: number) {
+        typeCheck(img);
         img.makeWritable()
         dx |= 0
         dy |= 0
@@ -276,6 +309,7 @@ namespace pxsim.ImageMethods {
     }
 
     export function replace(img: RefImage, from: number, to: number) {
+        typeCheck(img);
         to &= 0xf;
         const d = img.data
         for (let i = 0; i < d.length; ++i)
@@ -283,6 +317,7 @@ namespace pxsim.ImageMethods {
     }
 
     export function doubledX(img: RefImage) {
+        typeCheck(img);
         const w = img._width
         const h = img._height
         const d = img.data
@@ -300,6 +335,7 @@ namespace pxsim.ImageMethods {
     }
 
     export function doubledY(img: RefImage) {
+        typeCheck(img);
         const w = img._width
         const h = img._height
         const d = img.data
@@ -324,10 +360,13 @@ namespace pxsim.ImageMethods {
 
 
     export function doubled(img: RefImage) {
+        typeCheck(img);
         return doubledX(doubledY(img))
     }
 
     function drawImageCore(img: RefImage, from: RefImage, x: number, y: number, clear: boolean, check: boolean) {
+        typeCheck(img);
+        typeCheck(from);
         x |= 0
         y |= 0
 
@@ -441,6 +480,7 @@ namespace pxsim.ImageMethods {
     }
 
     export function drawLine(img: RefImage, x0: number, y0: number, x1: number, y1: number, c: number) {
+        typeCheck(img);
         x0 |= 0
         y0 |= 0
         x1 |= 0
@@ -525,6 +565,8 @@ namespace pxsim.ImageMethods {
     }
 
     export function drawIcon(img: RefImage, icon: RefBuffer, x: number, y: number, color: number) {
+        typeCheck(img);
+        BufferMethods.typeCheck(icon);
         const src: Uint8Array = icon.data
         if (!image.isValidImage(icon))
             return
@@ -586,6 +628,7 @@ namespace pxsim.ImageMethods {
     }
 
     export function fillCircle(img: RefImage, cx: number, cy: number, r: number, c: number) {
+        typeCheck(img);
         let x = r - 1;
         let y = 0;
         let dx = 1;
@@ -730,6 +773,7 @@ namespace pxsim.ImageMethods {
     }
 
     export function fillTriangle(img: RefImage, x0: number, y0: number, x1: number, y1: number, x2: number, y2: number, c: number) {
+        typeCheck(img);
         if (x1 < x0) {
             [x1, x0] = [x0, x1];
             [y1, y0] = [y0, y1];
@@ -785,6 +829,8 @@ namespace pxsim.ImageMethods {
     }
 
     export function _fillTriangle(img: RefImage, args: RefCollection) {
+        typeCheck(img);
+        Array_.typeCheck(args);
         fillTriangle(
             img,
             args.getAt(0) | 0,
@@ -858,6 +904,8 @@ namespace pxsim.ImageMethods {
     }
 
     export function blitRow(img: RefImage, x: number, y: number, from: RefImage, fromX: number, fromH: number) {
+        typeCheck(img);
+        typeCheck(from);
         x |= 0
         y |= 0
         fromX |= 0
@@ -885,6 +933,9 @@ namespace pxsim.ImageMethods {
     }
 
     export function blit(dst: RefImage, src: RefImage, args: RefCollection): boolean {
+        typeCheck(dst);
+        typeCheck(src);
+        Array_.typeCheck(args);
         const xDst = args.getAt(0) as number;
         const yDst = args.getAt(1) as number;
         const wDst = args.getAt(2) as number;
@@ -1064,6 +1115,10 @@ namespace pxsim.ImageMethods {
     }
 
     export function drawScaledRotatedImage(dst: RefImage, src: RefImage, args: RefCollection) {
+        typeCheck(dst);
+        typeCheck(src);
+        Array_.typeCheck(args);
+
         const xDst = args.getAt(0) as number;
         const yDst = args.getAt(1) as number;
         if (xDst >= dst._width || yDst >= dst._height) {
@@ -1127,6 +1182,10 @@ namespace pxsim.ImageMethods {
     }
 
     export function _checkOverlapsScaledRotatedImage(dst: RefImage, src: RefImage, args: RefCollection): boolean {
+        typeCheck(dst);
+        typeCheck(src);
+        Array_.typeCheck(args);
+
         const xDst = args.getAt(0) as number;
         const yDst = args.getAt(1) as number;
         if (xDst >= dst._width || yDst >= dst._height) {
@@ -1195,6 +1254,9 @@ namespace pxsim.ImageMethods {
     }
 
     export function _checkOverlapsTwoScaledRotatedImages(dst: RefImage, src: RefImage, args: RefCollection): boolean {
+        typeCheck(dst);
+        typeCheck(src);
+
         const xDst = args.getAt(0) as number;
         const yDst = args.getAt(1) as number;
         const dstArgs = parseShearArgs(dst, args, 2);
@@ -1339,6 +1401,12 @@ namespace pxsim.ImageMethods {
 
         return false;
     }
+
+    export function typeCheck(value: RefImage) {
+        if (!(value instanceof RefImage)) {
+            pxsim.throwFailedCastError(value, "Image");
+        }
+    }
 }
 
 
@@ -1400,6 +1468,7 @@ namespace pxsim.image {
     }
 
     export function ofBuffer(buf: RefBuffer): RefImage {
+        BufferMethods.typeCheck(buf);
         const src: Uint8Array = buf.data
 
         let srcP = 4
@@ -1460,6 +1529,7 @@ namespace pxsim.image {
     }
 
     export function toBuffer(img: RefImage): RefBuffer {
+        ImageMethods.typeCheck(img);
         let col = byteHeight(img._height, img._bpp)
         let sz = 8 + img._width * col
         let r = new Uint8Array(sz)
@@ -1503,6 +1573,7 @@ namespace pxsim.image {
     }
 
     export function doubledIcon(buf: RefBuffer): RefBuffer {
+        BufferMethods.typeCheck(buf);
         let img = ofBuffer(buf)
         if (!img)
             return null
@@ -1513,6 +1584,7 @@ namespace pxsim.image {
 
 namespace pxsim.pxtcore {
     export function updateScreen(img: RefImage) {
+        ImageMethods.typeCheck(img);
         const state = getScreenState();
         if (state)
             state.showImage(img)
@@ -1523,6 +1595,7 @@ namespace pxsim.pxtcore {
             state.updateStats(s);
     }
     export function setPalette(b: RefBuffer) {
+        BufferMethods.typeCheck(b);
         const state = getScreenState();
         if (state)
             state.setPalette(b)
@@ -1533,6 +1606,7 @@ namespace pxsim.pxtcore {
             state.setupScreenStatusBar(barHeight);
     }
     export function updateScreenStatusBar(img: RefImage) {
+        ImageMethods.typeCheck(img);
         const state = getScreenState();
         if (state)
             state.updateScreenStatusBar(img);
diff --git a/libs/settings/sim/settings.ts b/libs/settings/sim/settings.ts
index 6badf0c1a..b55ffa320 100644
--- a/libs/settings/sim/settings.ts
+++ b/libs/settings/sim/settings.ts
@@ -26,6 +26,7 @@ namespace pxsim.settings {
     }
 
     export function _set(key: string, buf: RefBuffer) {
+        BufferMethods.typeCheck(buf);
         key = encodeKey(key)
         const storage = board().storedState
         const prev = storage[key]
diff --git a/libs/shader/sim/shader.ts b/libs/shader/sim/shader.ts
index 9132a119d..2e215f52f 100644
--- a/libs/shader/sim/shader.ts
+++ b/libs/shader/sim/shader.ts
@@ -2,6 +2,8 @@
 
 namespace pxsim.ShaderMethods {
     export function _mergeImage(dst: RefImage, src: RefImage, xy: number) {
+        ImageMethods.typeCheck(dst);
+        ImageMethods.typeCheck(src);
         mergeImage(dst, src, pxsim.ImageMethods.XX(xy), pxsim.ImageMethods.YY(xy));
     }
 
@@ -19,6 +21,9 @@ namespace pxsim.ShaderMethods {
     }
 
     export function _mapImage(dst: RefImage, src: RefImage, xy: number, buf: RefBuffer) {
+        ImageMethods.typeCheck(dst);
+        ImageMethods.typeCheck(src);
+        BufferMethods.typeCheck(buf);
         mapImage(dst, src, pxsim.ImageMethods.XX(xy), pxsim.ImageMethods.YY(xy), buf);
     }
 
diff --git a/libs/storage/sim/storage.ts b/libs/storage/sim/storage.ts
index 42f6d0153..1720cbfea 100644
--- a/libs/storage/sim/storage.ts
+++ b/libs/storage/sim/storage.ts
@@ -5,6 +5,7 @@ namespace pxsim.storage {
     }
 
     export function appendBuffer(filename: string, data: RefBuffer): void {
+        BufferMethods.typeCheck(data);
         const state = storageState();
         let buf = state.files[filename];
         if (!buf) buf = state.files[filename] = [];
@@ -13,6 +14,7 @@ namespace pxsim.storage {
     }
 
     export function overwriteWithBuffer(filename: string, data: RefBuffer): void {
+        BufferMethods.typeCheck(data);
         const state = storageState();
         const buf = [];
         for (let i = 0; i < data.data.length; ++i)
diff --git a/libs/wifi---esp32/sim/wifisockets.ts b/libs/wifi---esp32/sim/wifisockets.ts
index 3599b62e4..ead1587c0 100644
--- a/libs/wifi---esp32/sim/wifisockets.ts
+++ b/libs/wifi---esp32/sim/wifisockets.ts
@@ -207,6 +207,7 @@ namespace pxsim._wifi {
     }
 
     export async function socketWrite(fd: int32, data: RefBuffer): Promise<int32> {
+        BufferMethods.typeCheck(data);
         const sock = getSock(fd)
         if (!sock)
             return -11
@@ -284,6 +285,7 @@ namespace pxsim._wifi {
 
 namespace pxsim.crypto {
     export function _sha256(bufs: RefCollection): Promise<RefBuffer> {
+        Array_.typeCheck(bufs);
         let len = 0
         const buffers = bufs.toArray().filter(e => e instanceof RefBuffer).map((b: RefBuffer) => {
             len += b.data.length