Updates on the supposedly minimum element aren't:

GHCi, version 6.8.2: http://www.haskell.org/ghc/  :? for help
Loading package base ... linking ... done.
Prelude> import Data.IntMap
Prelude Data.IntMap> let base = (insert 2 "b" (singleton 1 "a"))
Loading package array-0.1.0.0 ... linking ... done.
Loading package containers-0.1.0.1 ... linking ... done.
Prelude Data.IntMap> let umin = updateMinWithKey (\ k _ -> ("yo!" ++ (show k)) ) base
Prelude Data.IntMap> umin
fromList [(1,"a"),(2,"yo!2")]
Prelude Data.IntMap> showTree umin
"*\n+-- 1:=\"a\"\n+-- 2:=\"yo!2\"\n"

Updates on the maximum element are even worse, corrupting the data structure (ghci session continued):

Prelude Data.IntMap> let umax = updateMaxWithKey (\ k _ -> ("yo!" ++ (show k)) ) base
Prelude Data.IntMap> umax
fromList [(1,"yo!1"),(1,"a")]
Prelude Data.IntMap> showTree umax
"*\n+-- 1:=\"yo!1\"\n+-- 1:=\"a\"\n"

GHC is Debian 6.8.2-5.

Anything else that would be helpful? Thanks much in advance.