Measured Value Error

I am attempting to riff on the example widget for Bucketing by Gift Size, as presented in Take Your Tessitura Analytics Skills to the Next Level and I'm getting stuck due to an error I don't fully understand.

When I go to build out the filtered value formula using Contribution Count and Amount (just like at 13:10 in the video), I'm getting an error message which I think is telling me that the same measure is used more than once. My Contribution Amount field is coming up slightly differently than in his example, which makes me wonder if something got patched or otherwise changed.

Has anyone else encountered this and found a workaround? My end goal would be to have a count of donors giving $XX and up in a given year.