Errors accessing .net class method overloads in IronPython

Question

I have a class I've written in C#. The class has two methods, the signatures being:

bool Navigate(string url)
bool Navigate(Uri url)

From what I gather, the IronPython runtime is supposed to try and select the best overload based on the passed-in argument. In my case, I'm passing in a string which I know to be non-null, yet I get the following exception:

Multiple targets could match: Navigate(Uri), Navigate(str)

Seeing as my argument is blatantly a string, why does IronPython insist that multiple targets could match? System.String does not cast to System.Uri and as such the second method overload should not be a viable candidate for selection...