MozRepl gives ReferenceError sometimes (BrowserReload())

Question

I'm using MozRepl with Vim to reload the browser. This is the command I run:

echo "BrowserReload(); repl.quit()" | nc -w 1 localhost 4242

Sometimes it works, other times it gives ReferenceError: BrowserReload is not defined. Anybody in the know?

EDIT: OK, the problem is when the developer window is focused instead of the main window of Firefox.