diff --git a/Data/GroupedList.hs b/Data/GroupedList.hs
--- a/Data/GroupedList.hs
+++ b/Data/GroupedList.hs
@@ -236,7 +236,7 @@
 instance Foldable Grouped where
   foldMap f (Grouped gs) = foldMap (foldMap f) gs
 #if MIN_VERSION_base(4,8,0)
-  length (Grouped gs) = foldl' (+) 0 $ fmap length gs
+  length (Grouped gs) = foldl' (\s g -> s + length g) 0 gs
   null (Grouped gs) = null gs
 #else
 
diff --git a/grouped-list.cabal b/grouped-list.cabal
--- a/grouped-list.cabal
+++ b/grouped-list.cabal
@@ -1,5 +1,5 @@
 name:                grouped-list
-version:             0.2.1.3
+version:             0.2.1.4
 synopsis:            Grouped lists. Equal consecutive elements are grouped.
 description:
   Grouped lists work like regular lists, except for two conditions:
diff --git a/test/examples.hs b/test/examples.hs
--- a/test/examples.hs
+++ b/test/examples.hs
@@ -62,3 +62,5 @@
     =: [3,3,2,3]
   runIdentity (fmap snd $ GL.traverseGroupedByGroupAccum (\acc g -> Identity (acc+1, GL.map (+acc) $ GL.fromGroup g)) 0 [1,2,3,3,4])
     =: [1,3,5,5,7]
+  length ([1,2,2,3,4,4,4,5] :: Grouped Int)
+    =: 8
diff --git a/test/properties.hs b/test/properties.hs
--- a/test/properties.hs
+++ b/test/properties.hs
@@ -33,6 +33,7 @@
   [ prop "left mempty" $ \(G xs) -> GL.empty <> xs == xs
   , prop "right mempty" $ \(G xs) -> xs <> GL.empty == xs
   , prop "mappend" $ \(G xs) (G ys) (G zs) -> (xs <> ys) <> zs == xs <> (ys <> zs)
+  , prop "mappend length" $ \(G xs) (G ys) -> length xs + length ys == length (xs <> ys)
   , prop "index" $ \(G xs) ->
       let n = GL.length xs
       in  (\i ->