Comment 3 for bug 1000787

https://code.launchpad.net/~dooferlad/launchpad/postponed-is-done/+merge/106611 contains code that will prevent the x/0 problem from being evident (it just sets the % complete to 0 since an infinitely wide progress bar didn't appeal), but doesn't address the problem that the code that was crashing, shouldn't have got into that state in the first place.