This was unintentionally broken by a patch for a related issue: IGNORE_EXCEPTION_DETAIL should ignore the module name
and the unintended behavior you're seeing is an open issue here: doctest.IGNORE_EXCEPTION_DETAIL doesn't match when no detail exists
So it's a bug, according to me. Which is pretty good assurance it will get fixed, since I wrote doctest
to begin with ;-) In the meantime, you may want to try the patch attached to the 2nd bug report.
Followup: Last night I checked in a fix for this, which will appear in the next releases of Pythons 2.7, 3.3, and 3.4. Thanks for the nudge :-)