Alt+number does not switch terminal windows but gives me an unwanted Bash function instead

I am running Ubuntu 18.04.1. I want Alt+n (where n=1,2,3,4…) to switch between the respective terminal windows to speed up my workflow. However, when I do this in terminal, I get an output: (arg: n), where n is the number I press. My keyboard shortcuts are set so that Alt+n switches to the nth window, but I don’t see this effect working.

I understand that this has something to do with readline arguments but this is a function I currently don’t need, and so I want to remap this to switch between terminal windows somehow.

Thanks for your help folks!

P.S. I should mention this also has an effect on pressing Alt+PageUp/PageDown, I get: ~3 with Alt+PageUp and ;~3 with Alt+PageDown.