String reference not set to an instance of a String. Parameter name: s error

I'm (attempting) implementing a customer membership provider for an application (code from the custom SQL membership provider video) and upon creating a new user in with a standard create user wizard control in debug mode, I get this error:

 String reference not set to an instance of a String. Parameter name: s

I semi-understand why but can't quite get it. Here's the method that throws the error:

 

       ''' <summary>
        ''' Encode password.
        ''' </summary>
        ''' <param name="password">Password.</param>
        ''' <returns>Encoded password.</returns>< ...