remapping helm keybindings

[88-percent]

Remapped helm keybindings don't stick until I've reloaded my configuration.

I've added the following to my user-config

(define-key helm-map (kbd "C-;") 'helm-toggle-visible-mark)

but trying to invoke the command returns:
[C-; is undefined]

C-; does not appear in the command list after invoking describe-keymap helm-map

Bizarrely, reloading my config (space f e R) fixes the problem, but only until emacs is shutdown. describe-keymap helm-map shows C-; as correctly assigned to helm-toggle-visible-mark. The shortcut then works as expected.

I've also tried wrapping the function in with-eval-after-load 'helm but the issue persists.

Not sure if this is a bug or user error. If the latter, any direction is appreciated.