String expected 

You attempted to invoke the String.prototype.toString or String.prototype.valueOf method on an object of a type other than String. The object of this type of invocation must be of type String.

To correct this error

• Only invoke the String.prototype.toString or String.prototype.valueOf methods on objects of type String.

See Also

Reference

String Object (JScript 5.6)
toString Method (JScript 5.6)