diff --git a/Diff.cabal b/Diff.cabal
--- a/Diff.cabal
+++ b/Diff.cabal
@@ -1,5 +1,5 @@
 name:                Diff
-version:             0.3.3
+version:             0.3.4
 synopsis:            O(ND) diff algorithm in haskell.
 description:         Implementation of the standard diff algorithm, and utilities for pretty printing.
 category:            Algorithms
diff --git a/src/Data/Algorithm/DiffOutput.hs b/src/Data/Algorithm/DiffOutput.hs
--- a/src/Data/Algorithm/DiffOutput.hs
+++ b/src/Data/Algorithm/DiffOutput.hs
@@ -15,6 +15,7 @@
 import Text.PrettyPrint
 import Data.Char
 import Data.List
+import Data.Monoid (mappend)
 
 -- | Converts Diffs to DiffOperations
 diffToLineRanges :: [Diff [String]] -> [DiffOperation LineRange]