One of the gotchas for Extension Methods is that it's legal to call them on Null References. This isn't really surprising when you think about the feature. Boiled down to a fundamental level, extension methods are just syntactic sugar for calling a static method and automatically passing the first parameter . However it can catch the unwary off guard.
The two items that are a little bit different between calling an Instance vs Extension method on a null reference is
Public Function IsNothing(ByVal o As Object) As Boolean
Return o Is Nothing
Dim x As String = Nothing
Dim b = x.IsNothing() ' b = True
This is legal and will not throw. However I don't recomend that you write it.
 Then again you could also claim that instance methods are just syntactic sugar for calling static methods without having to pass this/Me as the first parameter.
 In some ways this is similar to C++. C++ doesn't do NULL reference checking automatically (it waits for you to access data on a NULL reference and then crashes if you're lucky). If you call a method on a NULL pointer but don't actually access any member variables, it will likely run fine.