diff --git a/bson-mapping.cabal b/bson-mapping.cabal
--- a/bson-mapping.cabal
+++ b/bson-mapping.cabal
@@ -1,10 +1,10 @@
 Name:               bson-mapping
-Version:            0.1.2
+Version:            0.1.3
 License:            OtherLicense
 License-File:       LICENSE
 Author:             Francesco Mazzoli <f@mazzo.li>
 Maintainer:         Francesco Mazzoli <f@mazzo.li>
-homepage:           
+homepage:
 Stability:          experimental
 Category:           Data
 Synopsis:           Mapping between BSON and algebraic data types.
diff --git a/src/Data/Bson/Mapping.hs b/src/Data/Bson/Mapping.hs
--- a/src/Data/Bson/Mapping.hs
+++ b/src/Data/Bson/Mapping.hs
@@ -13,13 +13,14 @@
 
 > import Data.Bson.Mapping
 > import Data.Time.Clock
+> import Data.Data (Data, Typeable)
 >
 > data Post = Post { time :: UTCTime
 >                  , author :: String
 >                  , content :: String 
 >                  , votes :: Int
 >                  }
->           deriving (Show, Read, Eq, Ord)
+>           deriving (Show, Read, Eq, Ord, Data, Typeable)
 > $(deriveBson ''Post)
 >
 > main :: IO ()
@@ -140,10 +141,11 @@
     deriveFromBson conss = do
       con <- newName "con"
       docN <- newName "doc"
+      (SigE _ (ConT strtype)) <- runQ [| "" :: String |]
       let doc = varE docN
       lamE [varP docN] $ doE
         [ bindS (varP con) [| lookup consField $doc |]
-        , noBindS $ caseE (varE con) (map (genMatch doc) conss ++ noMatch)
+        , noBindS $ caseE (sigE (varE con) (conT strtype)) (map (genMatch doc) conss ++ noMatch)
         ]
  
     noMatch = [match [p| _ |] (normalB [| fail "Couldn't find right constructor" |]) []]