diff --git a/lib/internal/crypto/webidl.js b/lib/internal/crypto/webidl.js
index 9f5340c223902c..61a4cfe4330703 100644
--- a/lib/internal/crypto/webidl.js
+++ b/lib/internal/crypto/webidl.js
@@ -184,7 +184,10 @@ function isNonSharedArrayBuffer(V) {
 }
 
 function isSharedArrayBuffer(V) {
-  return ObjectPrototypeIsPrototypeOf(SharedArrayBuffer.prototype, V);
+  // SharedArrayBuffers can be disabled with --no-harmony-sharedarraybuffer.
+  if (SharedArrayBuffer !== undefined)
+    return ObjectPrototypeIsPrototypeOf(SharedArrayBuffer.prototype, V);
+  return false;
 }
 
 converters.Uint8Array = (V, opts = kEmptyObject) => {