Error when creating a custom rule [C7,]

I went to create a custom rule to send me an alert if the water sensor I have down my sump pump detected water (I was thinking to separate this from the other water alerts, which are critical, to this, which is just informative).

I selected the detect, added a text alert, and pressed "Done" (with the rule still unarmed), and got this error. The rule was still added, however.

This is the custom rule:

FWIW, I deleted the custom rule and then went to recreate it. I couldn't reproduce the error. This was my first (and only) custom HSM rule.