Field Validation error message

Hi ,

I am trying to make a field required based on a pick list value of another field. For Ex if the users picks a value from Field A, then he needs to pick a value from Field B. I tried putting this formula in the field validation of Field B

[<FieldB>] IS NULL AND FIELDA= (LookupValue("OCC_CUST_LOV_SR_0", "Machine")) Then Display the err message.

Its not working, its letting the user save the record though he doesn't pick any value from field B.

Any help is appreciated.