Unable to create Guest Account after enabling FileVault

Question

After upgrading to Lion, I tried creating a guest account and it worked.

I enabled FileVault on Lion and went through the process of encryption. After that, when I am trying to create a Guest account, the option itself is disabled.

I am unable to create a Guest account on Lion . I wanted to check if this is the usual behavior with FileVault enabled Macs since I could not find any related articles on this .