diff --git a/src/Signer/Ecdsa.php b/src/Signer/Ecdsa.php
index ff2f88e3..05f5210a 100644
--- a/src/Signer/Ecdsa.php
+++ b/src/Signer/Ecdsa.php
@@ -38,7 +38,7 @@ final protected function guardAgainstIncompatibleKey(int $type, int $lengthInBit
         if ($type !== OPENSSL_KEYTYPE_EC) {
             throw InvalidKeyProvided::incompatibleKeyType(
                 self::KEY_TYPE_MAP[OPENSSL_KEYTYPE_EC],
-                self::KEY_TYPE_MAP[$type],
+                self::KEY_TYPE_MAP[$type] ?? 'unknown',
             );
         }
 
diff --git a/src/Signer/Rsa.php b/src/Signer/Rsa.php
index 6a6dd5c8..0bf44d6a 100644
--- a/src/Signer/Rsa.php
+++ b/src/Signer/Rsa.php
@@ -24,7 +24,7 @@ final protected function guardAgainstIncompatibleKey(int $type, int $lengthInBit
         if ($type !== OPENSSL_KEYTYPE_RSA) {
             throw InvalidKeyProvided::incompatibleKeyType(
                 self::KEY_TYPE_MAP[OPENSSL_KEYTYPE_RSA],
-                self::KEY_TYPE_MAP[$type],
+                self::KEY_TYPE_MAP[$type] ?? 'unknown',
             );
         }