Dynamic Filter in Report - tricking it on a formula text?

I might make a formula text field, where say it checks a date and either returns 'Current Month', 'Previous Month', or else left blank.

If I use that formula as a dynamic filter, the  3 choices are exactly that: Current Month, Previous Month, Blank.

Any sneaky way to get rid of the 'blank' option? So they can choose only current month or previous month to filter? I could do an 'ask the user' filter on the date, but I really want it dynamic. I could make my formula text have else be 'all other months'. But would love a sneaky trick to get rid of the blank option.

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Mike Tamoush
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