Comment #3 on issue 507 by manpac...@gmail.com: Pattern match failure in do expression at XMonad/Prompt.hs:601:3-14 http://code.google.com/p/xmonad/issues/detail?id=507 Right, the problem is that lookupString already returns keypresses which are already in proper unicode and whoever implemented utf8-string library decided not to use any checks if string is in unicode format already and some other guy who implemented Xmonad.Prompt decided that he can assume that color passed into initColor will be always correct and yet one more guy decided that catching any exceptions inside that initColor function and returning Nothing should be safe - because of all this (and some haskell lazyness) we have a nice, clean and 100% unrelated error message about this pattern match. So if you are affected by that bug and looking for a quick fix - just go into XMonad.Prompt module, look for keyPressHandle function, look for decodeString near insertString call and just remove that decodeString part.