Can't figure out how to get tool tips to work

I’ve been experimenting with adding a new technology to the Stable building… the technology appears as a button, but once I hover over the button, no tool tip appears (that is, a tool tip that shows the name of the technology, its cost, what it does, etc).

I have added all the necessary strings under key-value-modded-strings-utf8 (in resources\en\strings\key-value).

Anyone have a clue what’s the issue here?