Alt stopped working in rxvt-unicode

My Alt key stopped working in rxvt-unicode. Still works in Firefox. I don’t have any idea about how it stopped working. After some searching and trying, I found out that I had to add a line to my .Xdefaults file.

URxvt.modifier: meta

After reloading the file with xrdb -merge ~/.Xdefaults, the Alt key started working again as it did before.