Capital M is behaving weirdly in insert-state of evil-mode

Question

I am observing a strange behavior in evil-mode that I'm unable to figure out what is causing it: In insert-state, pressing "M" does not insert a capital m as expected, but instead gives me the following prompt in the minibuffer (see screenshot 1 below). Strangely, "M" works in visual-state and normal-mode. I have double checked my init.el and I have not accidentally bound M to anything, and all other letters seem to work.

What kind of mode is this showing and how can I debug this?

Update: I tried C-h k M in insert-state, but this just gave me this:

Update: Turns out this line was the culprit:

(define-key evil-insert-state-map "M-r" 'evil-paste-from-register)

Is this a bug in evil-mode?

Answer 1

In answer to your 'update',

You should use an escape character before the "M" as explained in the Changing Key Bindings section of the elisp manual.

Alternatively, you could wrap the "M-r" in a kbd form as explained in the Key Sequences section of the elisp manual.

(evil-paste-from-register is bound to C-r by default in evil insert state, but I guess you are aware of that already. Just to be sure...)