Having problems with boost::signal and boost::function

I'm having a problem (which was really hard to decipher) when connecting a function object to a signal. Thing is I default construct (no argument) a function object and connect it to the signal (using boost::ref), then at a later point in time I actually assign something to the function object (using boost::bind). But firing the signal after this causes a bad_function_call to be thrown. On the other hand; if I construct the function object with some dummy object before connecting it to the signal, and later do the assignment, the signal does its thing. If someone could enlighten me as to whats going on here, I'd be grateful .. : )