diff --git a/Language/SMTLib2/Pipe.hs b/Language/SMTLib2/Pipe.hs
--- a/Language/SMTLib2/Pipe.hs
+++ b/Language/SMTLib2/Pipe.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE ViewPatterns #-}
+{-# LANGUAGE ViewPatterns, ImpredicativeTypes #-}
 module Language.SMTLib2.Pipe
        (SMTPipe(),
         FunctionParser(..),
@@ -317,7 +317,7 @@
     Just (Just x) -> return (x,pipe)
     _ -> error $ "smtlib2: Couldn't parse proof "++show res
   where
-    findProof [] = Nothing
+    findProof [] = Nothing :: Maybe L.Lisp
     findProof ((L.List [L.Symbol "proof",proof]):_) = Just proof
     findProof (x:xs) = findProof xs
 handleRequest pipe SMTGetUnsatCore = do
@@ -378,7 +378,7 @@
         Just rtp' -> let argMp = Map.fromList [ (name,(i,sort))
                                               | (i,(name,sort)) <- zip [0..] args' ]
                          funId = case unescapeName (T.unpack fname) of
-                           Nothing -> Nothing
+                           Nothing -> Nothing :: Maybe Integer
                            Just (Right idx) -> Just idx
                            Just (Left name) -> case Map.lookup name (namedVars $ smtState pipe) of
                              Just idx -> Just idx
@@ -470,18 +470,10 @@
               -> [String] -- ^ Command line arguments to be passed to the SMT solver
               -> IO SMTPipe
 createSMTPipe solver args = do
-  let cmd = CreateProcess { cmdspec = RawCommand solver args
-                          , cwd = Nothing
-                          , env = Nothing
-                          , std_in = CreatePipe
-                          , std_out = CreatePipe
-                          , std_err = Inherit
-                          , close_fds = False
-                          , create_group = True
-#if MIN_VERSION_process(1,2,0)
-                          , delegate_ctlc = False
-#endif
-                          }
+  let cmd = (proc solver args) { std_in = CreatePipe
+                               , std_out = CreatePipe
+                               , std_err = Inherit
+                               , create_group = True }
   (Just hin,Just hout,_,handle) <- createProcess cmd
   return $ SMTPipe { channelIn = hin
                    , channelOut = hout
@@ -1272,7 +1264,7 @@
               ,fun]) rec dts
 #ifdef SMTLIB2_WITH_CONSTRAINTS
       = case parseFun rec fun rec dts of
-        Nothing -> Nothing
+        Nothing -> Nothing :: Maybe FunctionParser'
         Just (DefinedParser _ ret_sig parse)
           -> Just $ OverloadedParser
             { sortConstraint = const True
diff --git a/smtlib2.cabal b/smtlib2.cabal
--- a/smtlib2.cabal
+++ b/smtlib2.cabal
@@ -1,5 +1,5 @@
 Name:           smtlib2
-Version:        0.3
+Version:        0.3.1
 Author:         Henning Günther <guenther@forsyte.at>
 Maintainer:     guenther@forsyte.at
 Synopsis:       A type-safe interface to communicate with an SMT solver.