I am working on the "Task" fields for  "Activities" and trying to create a validation rule based on two different picklist values.

The following is my formula, but I have no idea how to keep from getting errors........

AND(ISPICKVAL( Subject ,"AOP"),(ISPICKVAL( AOP_Task_Type__c ,"")))

Error: Incorrect parameter for function ISPICKVAL(). Expected Picklist, received Text

I have tried many different ways to get this to work with no success....

Scenario: If "AOP" is chosen as the subject, an AOP_Task_Type field is required. (Both fields are picklists)

Any help would be greatly appreciated....

Thanks