nnoremap j gj not working from vimrc

I've tried placing the following:

nnoremap j gj
nnoremap k gk

in the vimrc, but is causes some funny behaviour.
I'm using gvim, and when I issue the commands above manually, they do what I expect, the cursor moves up/down one visual line at a time.

When placed in the vimrc, though, when I start up gvim and try navigating, it does some unexpected stuff. j moves down 1 and over 1, but skips empty lines. k moves the cursor up one and over 1, but gets stuck on empty lines.

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