When throwing an Error whose message contains the name of a function, the JS engine calls js_ValueToFunction to produce the function name to include in the message. js_ValueToFunction does not unwrap its argument. So if the function is actually a wrapper, then js_ValueToFunction will throw a TypeError ("X is not a function"). Confusingly, this TypeError uses the decompiler, which *will* unwrap the object to get the wrapped function name out, so the final error will look something like "SomeFunction() is not a function".) Either neither or both should unwrap.