Filtering custom fields that are not set out of custom queries

I have a custom issue field that I need to query on, but I would like to select all issues where the custom field is set. I can't seem to find a "none" or "is not none" filter. The only filters I see are "is", "contains", "is not", and "doesn't contain" and all of them expect some sort of value. Anyone know a way to get around this?