diff --git a/prolog.cabal b/prolog.cabal
--- a/prolog.cabal
+++ b/prolog.cabal
@@ -1,5 +1,5 @@
 Name:                prolog
-Version:             0.2.1
+Version:             0.2.1.1
 Synopsis:            A Prolog interpreter written in Haskell.
 Description:         A Prolog interpreter written in Haskell. Implements a subset of the Prolog language.
 License:             PublicDomain
diff --git a/src/Database.hs b/src/Database.hs
--- a/src/Database.hs
+++ b/src/Database.hs
@@ -10,7 +10,7 @@
 where
 
 import Data.Map (Map)
-import qualified Data.Map as Map
+import qualified Data.Map.Strict as Map
 
 import Syntax
 
@@ -28,7 +28,7 @@
 hasPredicate sig (DB index) = Map.member sig index
 
 createDB clauses emptyPredicates = DB $
-   foldr (\clause -> Map.insertWith' (++) (signature (lhs clause)) [clause])
+   foldr (\clause -> Map.insertWith (++) (signature (lhs clause)) [clause])
          (Map.fromList [ (signature (Struct name []), []) | name <- emptyPredicates ])
          clauses