Build failure with mtl-2.3

[sjakobi]

Building library for libgit-0.3.1..
[1 of 5] Compiling Lib.Git.Type     ( Lib/Git/Type.hs, dist/build/Lib/Git/Type.o, dist/build/Lib/Git/Type.dyn_o )

Lib/Git/Type.hs:152:9: error:
    Variable not in scope: when :: Bool -> IO () -> IO a0
    |
152 |         when (length input > 0) $ do hPutStr inh input; hFlush inh
    |         ^^^^