diff --git a/test/Web/HttpApiData/Internal/FormUrlEncodedSpec.hs b/test/Web/HttpApiData/Internal/FormUrlEncodedSpec.hs
index 0520200..2ba009e 100644
--- a/test/Web/HttpApiData/Internal/FormUrlEncodedSpec.hs
+++ b/test/Web/HttpApiData/Internal/FormUrlEncodedSpec.hs
@@ -29,15 +29,33 @@ genericSpec = describe "Default (generic) instances" $ do
       M.lookup "rec1" f `shouldBe` Just (rec1 x)
       (parseUrlPiece <$> M.lookup "rec2" f) `shouldBe` Just (Right $ rec2 x)
 
+    context "for sum types" $ do
+
+      it "contains the correct records" $ property $ \x  -> do
+        let f = unForm $ toForm x
+        case x of
+          SSRLeft _ _ -> do
+            parseUrlPiece <$> M.lookup "left1" f `shouldBe` Just (Right $ left1 x)
+            parseUrlPiece <$> M.lookup "left2" f `shouldBe` Just (Right $ left2 x)
+          SSRRight _ _ -> do
+            parseUrlPiece <$> M.lookup "right1" f `shouldBe` Just (Right $ right1 x)
+            parseUrlPiece <$> M.lookup "right2" f `shouldBe` Just (Right $ right2 x)
+
+
   context "FromForm" $ do
 
-    it "is the left inverse of ToForm" $ property $ \(x :: SimpleRec) -> do
-      fromForm (toForm x) `shouldBe` Right x
+    it "is the left inverse of ToForm" $ property $
+      \(x :: SimpleRec, y :: SimpleSumRec) -> do
+        fromForm (toForm x) `shouldBe` Right x
+        fromForm (toForm y) `shouldBe` Right y
 
     it "is the right inverse of ToForm" $ property $ \x (y :: Int) -> do
-      let f = Form $ M.fromList [("rec1", x), ("rec2", toUrlPiece y)]
-          Right r = fromForm f :: Either String SimpleRec
-      toForm r `shouldBe` f
+      let f1 = Form $ M.fromList [("rec1", x), ("rec2", toUrlPiece y)]
+          Right r1 = fromForm f1 :: Either String SimpleRec
+      toForm r1 `shouldBe` f1
+      let f2 = Form $ M.fromList [("right1", x), ("right2", toUrlPiece y)]
+          Right r2 = fromForm f2 :: Either String SimpleSumRec
+      toForm r2 `shouldBe` f2
 
     it "returns the underlying error" $ do
       let f = Form $ M.fromList [("rec1", "anything"), ("rec2", "bad")]
diff --git a/test/Web/HttpApiData/Internal/TestInstances.hs b/test/Web/HttpApiData/Internal/TestInstances.hs
index e39f919..d72437e 100644
--- a/test/Web/HttpApiData/Internal/TestInstances.hs
+++ b/test/Web/HttpApiData/Internal/TestInstances.hs
@@ -5,6 +5,7 @@
 module Web.HttpApiData.Internal.TestInstances
    ( RandomCase(..)
    , SimpleRec(..)
+   , SimpleSumRec(..)
    ) where
 
 import           Control.Applicative
@@ -81,3 +82,17 @@ instance FromForm SimpleRec
 
 instance Arbitrary SimpleRec where
   arbitrary = SimpleRec <$> arbitrary <*> arbitrary
+
+data SimpleSumRec
+  = SSRLeft { left1 :: Int, left2 :: Bool }
+  | SSRRight { right1 :: T.Text, right2 :: Int}
+  deriving (Eq, Show, Read, Generic)
+
+instance ToForm SimpleSumRec
+instance FromForm SimpleSumRec
+
+instance Arbitrary SimpleSumRec where
+  arbitrary = oneof
+    [ SSRLeft <$> arbitrary <*> arbitrary
+    , SSRRight <$> arbitrary <*> arbitrary
+    ]