PasswordReset() throws exception - passwordAnswer?

I have an app that I'm trying to implement the following, but can't seem to figure out how:

1. User signs up, but the system creates its own temporary password for them
2. Admin approves user and as part of that approval, the user gets sent a temporary username/password.

The problem is that I can't set the MembershipProvider to enable password retrieval as that seems to disable certficate authentication. I do have passwordReset enabled, but when I try to use it in step 2 (trying to create a new password so I can have it in plaintext to email it to the user), it throws an error:

General Error: System.Web.HttpUnhandledException (0x80004005): Exception of type 'System.Web.HttpUnhandledException' was thrown. ---> System.ArgumentNullException: Value cannot be null. Parameter name: passwordAnswer

Is there any way around this?

Here's a code snippet of the relevant code:

MembershipUser mu = Membership.GetUser(Session["UserId"], false);
string password = mu.ResetPassword(); 

The password is null because nothing got entered. In your code, just use an if:

if (passwordAnswer==null){
//do stuff here
}

While you can write anything there, I recommend, maybe:

if (passwordAnswer==null){
 passwordAnswer=" ";//just a space
}

If you want it to be a generated password, research on generating random alphanumeric strings.