"Null" cannot be used in the criteria for a rule!!!

"Null" cannot be used in the criteria for a rule!!!

Hi,

I would like to create a rule that if a value of a lookup field has not been selected then it will hide add entries of a subform.

When I try to create the rule I get this error "Null can not be used in the criteria for a rule."

Please explain how to create a rule for an empty value of a lookup field if criteria cannot be "Null"???

Thanks,
Damien Cregan
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