forms based authentication asp 4.0

Hi all,
I have the following code setup for forms based authentication with an ms sql backend that accepts a username and password for verification.  The problem that I'm running into is when I click the "log in" button after I enter credentials.  I get the following error:

Object reference not set to an instance of an object. Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.
Source Error:

Line 89: Line 90: Line 91: if (ValidateUser(Us ...