WCF / ASMX :: Publish Enum Through Web Service Without Being Param/return Type?

I have a webmethod that has three parameters:

public AuthenticationResult Authenticate(Enum type, string userName, string password) But the problem I had is that if a null/invalid enum is entered for the type variable it throws an error message.

So the obvious solution is to use a string/int instead but I am cannot do this as current users already use the existing webmethod.

So i was thinking:

public AuthenticationResult Authenticate(object type, string userName, string password) and handling the casting myself but the problem I had with this is that now my users do not have access to the Enum type.

So my question is can I publish the enum type without having it as a parameter/return value? Allow the client to access the Enum so they can pass it into the method. Would using a method that returned a type that was my enum work? Or is there a way to catch this custom error: (I dont think so as it appears to be on the client end when converting Enum to xml). The error is: (System.InvalidOperationException: There was an error generating the XML document. ---> System.InvalidOperationException: Instance validation error: '45465' is not a valid value for Enum.)