Ambiguous call between two C# extension generic methods one where T:class and other where T:struct

Consider two extension methods:

public static T MyExtension<T>(this T o) where T:class
public static T MyExtension<T>(this T o) where T:struct

And a class:

class MyClass() { ... }

Now call the extension method on a instance of the above class:

var o = new MyClass(...);
o.MyExtension(); //compiler error here..
o.MyExtension<MyClass>(); //tried this as well - still compiler error..

The compiler says that calling the method is an ambiguous call when I call it on a class. I would have thought that it could determine which extension method to call, as MyClass is a class, not a struct?