diff --git a/src/utils.ts b/src/utils.ts
index f5be2f1be2..e64a450ff3 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -80,6 +80,11 @@ const enum CharacterCode {
  * @returns Indicates if `str` is HTML.
  */
 export function isHtml(str: string): boolean {
+
+  if(typeof str !== 'string') {
+    return false;
+  }
+
   const tagStart = str.indexOf('<');
 
   if (tagStart === -1 || tagStart > str.length - 3) return false;