Picklist Value required based on Checkbox

Hello,

I am trying to make a field required if a check box is selected, but I can't seem to get it right. The check box is called "Is_Publisher_Currently_Live__c" and the field is called "AdSize_728x90__c."

Below is what I created. The problem is that the validation is not recognizing the "--None--" value of the picklist, since it is a default addition to the picklist. As per someone's suggestion, I also tried to replace "--None--" with "empty," but to no avail. Please help!

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AND(
Is_Publisher_Currently_Live__c,
ISPICKVAL( AdSize_728x90__c, "--None--")
)

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Thanks,

Noah