Can't get the increase binding to work

[level39]

First, it doesn't support the default font format:

URxvt.font: 9x15,xft:TerminessTTFNerdFontMono

So removing 9x15, and appending :size=12, you get the decrease and reset bindings working but the increase binding won't work (it doesn't warn either which means it might have succeeded in registering). So, pressing C-plus just prints the equal sign.

Is there a workaround or a binding that works?