diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,3 +1,7 @@
+## 1.2.6.4
+
+* Fix benchmark by adding a type signature
+
 ## 1.2.6.3
 
 * Doc updates
diff --git a/benchmarks/optimize-201408.hs b/benchmarks/optimize-201408.hs
--- a/benchmarks/optimize-201408.hs
+++ b/benchmarks/optimize-201408.hs
@@ -166,7 +166,8 @@
         return $ fromIntegral successes / fromIntegral count * 4
     , TBIOTest "low level" closeEnough $ do
         gen <- MWC.createSystemRandom
-        let go 0 !t = return $! fromIntegral t / fromIntegral count * 4
+        let go :: Int -> Int -> IO Double
+            go 0 !t = return $! fromIntegral t / fromIntegral count * 4
             go i !t = do
                 (x, y) <- MWC.uniform gen
                 let t'
diff --git a/conduit.cabal b/conduit.cabal
--- a/conduit.cabal
+++ b/conduit.cabal
@@ -1,5 +1,5 @@
 Name:                conduit
-Version:             1.2.6.3
+Version:             1.2.6.4
 Synopsis:            Streaming data processing library.
 description:
     Hackage documentation generation is not reliable. For up to date documentation, please see: <http://www.stackage.org/package/conduit>.