When pressing `:` on an empty line with `set debug+=msg`, get error messages

[axvr]

When using traces.vim with set debug+=msg set, I get error messages when pressing : on an empty line. This makes debugging other Vim scripts more difficult and annoying. I assume this is indicative of a bug in traces.vim.

Reproduce:

1. Open Vim,
2. :set debug+=msg
3. Press : in normal mode.