diff --git a/AesonBson.cabal b/AesonBson.cabal
--- a/AesonBson.cabal
+++ b/AesonBson.cabal
@@ -1,5 +1,5 @@
 name:          AesonBson
-version:       0.2.0
+version:       0.2.1
 license:       OtherLicense
 license-file:  LICENSE
 copyright:     CC0
@@ -7,10 +7,9 @@
 maintainer:    Niklas Hambüchen <mail@nh2.me>
 category:      Data
 build-type:    Simple
-synopsis:      Python-to-Haskell function calls
 stability:     experimental
 tested-With:   GHC==7.4.2
-cabal-version: >= 1.10
+cabal-version: >= 1.8
 homepage:      https://github.com/nh2/AesonBson
 bug-Reports:   https://github.com/nh2/AesonBson/issues
 synopsis:      Mapping between Aeson's JSON and Bson objects.
@@ -34,5 +33,22 @@
     unordered-containers >= 0.1.3.0,
     vector >= 0.7.1
   hs-source-dirs: .
-  default-language: Haskell2010
+  ghc-options: -Wall
+
+
+
+test-Suite tests
+  type: exitcode-stdio-1.0
+  hs-source-dirs:
+    test
+  main-is:
+    Main.hs
+  build-depends:
+    base < 5,
+    AesonBson,
+    aeson,
+    bson,
+    hspec >= 1.7.2.1,
+    HUnit >= 1.2.5.2,
+    text >= 0.11.3.1
   ghc-options: -Wall
diff --git a/Data/AesonBson.hs b/Data/AesonBson.hs
--- a/Data/AesonBson.hs
+++ b/Data/AesonBson.hs
@@ -24,7 +24,6 @@
 import           Data.Monoid
 import qualified Data.HashMap.Strict as HashMap (fromList, toList)
 import qualified Data.Vector as Vector (fromList, toList)
-import           Numeric (showHex)
 
 
 -- | Converts a JSON value to BSON.
@@ -49,7 +48,9 @@
 aesonifyValue (Uuid (UUID uuid)) = toJSON uuid
 aesonifyValue (Md5 (MD5 md5)) = toJSON md5
 aesonifyValue (UserDef (UserDefined userdef)) = toJSON userdef
-aesonifyValue (ObjId (Oid w32 w64)) = toJSON $ showHex w32 (showHex w64 "")
+aesonifyValue (ObjId oid) = toJSON $ show oid -- Relies on bson to show the OID as 24 digit hex.
+                                              -- It would be better if BSON exposed a non-show function for this,
+                                              -- preferably a fast one.
 aesonifyValue (BSON.Bool bool) = toJSON bool
 aesonifyValue (UTC utc) = toJSON utc
 aesonifyValue (BSON.Null) = AESON.Null
diff --git a/test/Main.hs b/test/Main.hs
new file mode 100644
--- /dev/null
+++ b/test/Main.hs
@@ -0,0 +1,21 @@
+{-# LANGUAGE OverloadedStrings #-}
+
+module Main where
+
+import           Test.Hspec
+import           Data.Aeson.Types as AESON
+import           Data.Bson as BSON
+import qualified Data.Text as T
+
+import Data.AesonBson
+
+main :: IO ()
+main = hspec $ do
+  describe "BSON -> JSON" $ do
+
+    it "converts an ObjId to 24 digits hex" $ do
+      -- https://github.com/nh2/aesonbson/pull/2
+      let objid = ObjId (read "000000010000000000000001" :: ObjectId)
+          AESON.String str = aesonifyValue objid
+      str `shouldBe` "000000010000000000000001"
+      T.length str `shouldBe` 24