JS5023: Function does not have a valid prototype object
The code uses instanceof to determine if an object was derived from a particular function class, but the code redefined the object's prototype property as either null or an external object type (both not valid JScript objects). An external object can be an object from the host object model (for example, Internet Explorer's document or window object), or an external COM object.
To correct this error
Ensure the function's prototype property refers to a valid JScript object.