Windows Authentication inside custom WebPart

Question

I have created a custom web part that let's the user sign in using their windows accounts. However, the I get Object Not Set to the Reference error when I set the SetPrincipalAndWriteSessionToken. Here is the code:

// Get Token
                var stoken = SPFormsUserNameSecurityTokenHandler.CreateSecurityToken(WindowsMembershipProvider, WindowsRoleProvider, username, password);
                if (stoken != null)
                {
                    // Sign User In
                    SPFederationAuthenticationModule fam = SPFederationAuthenticationModule.Current;   
                    fam.SetPrincipalAndWriteSessionToken(stoken);
                }

The WindowsMembershipProvider is "i", from the Web.Config and WindowsRoleProvider is "c" from the web.config. If I remove the roleprovider, it says "Value cannot be null", even though it is not a required parameter.

Can someone please help me out?

Thanks!