Help with Field Validation syntax

Hi,

I'm trying to write a field validation rule that won't allow the email field on the contact form to be left NULL once a value was entered, but exclude a specific contact type from this rule.

I have this syntax:

(PRE('<ContactEmail>') IS NULL AND [<ContactType>]<> LookupValue("CONTACT_TYPE", "Contact"))

the 'PRE' portion of the validation works, however its not taking into account the contact type value and prompts me even when the contact type is Contact.

Any help would be appreciated.

Jo-Ann