diff --git a/src/THInstanceReification.hs b/src/THInstanceReification.hs
--- a/src/THInstanceReification.hs
+++ b/src/THInstanceReification.hs
@@ -1,4 +1,3 @@
-{-# LANGUAGE CPP #-}
 -- |
 -- Fixed versions of 'reifyInstances' and 'isInstance' as per
 -- the following ghc issue:
@@ -41,14 +40,14 @@
     -- Expand type synonyms in type signatures, 
     -- using 'expandSyns' from the "th-expand-syns" library:
     expandedTypes <- mapM expandSyns tl
-    expendedInstanceTypes <- mapM expandSyns htl
+    expandedInstanceTypes <- mapM expandSyns htl
     maybe 
       (fail $ "Unmatching amounts of types: " <> show expandedTypes <> ", " <> 
-              show expendedInstanceTypes)
+              show expandedInstanceTypes)
       (analyze context)
       -- 'pair' is a safe version of 'zip' from the "list-extras" library,
       -- which returns 'Nothing', when lists differ in size.
-      (pair expandedTypes expendedInstanceTypes)
+      (pair expandedTypes expandedInstanceTypes)
   d -> fail $ "Not an instance dec: " <> show d
   where
     -- |
@@ -60,10 +59,8 @@
         -- |
         -- A partial function, 
         -- which returns a tested type by a variable name.
-        actualTypeByVarName :: Name -> Type
-        actualTypeByVarName = \n ->
-          Map.lookup n m ?: 
-          (error $ "Unexpected key: " <> show n <> ", in a map: " <> show m)
+        actualTypeByVarName :: Name -> Maybe Type
+        actualTypeByVarName = \n -> Map.lookup n m
           where
             -- A memoization cache.
             m = Map.fromList $ concat $ map accRecords $ typeAssocs
@@ -75,25 +72,18 @@
                 accRecords = \case
                   (AppT al ar, AppT hl hr) -> accRecords (al, hl) ++ accRecords (ar, hr)
                   (a, VarT n) -> [(n, a)]
-                  (a, h) | a /= h -> error $ "Unmatching types: " <> show a <> ", " <> show h
                   _ -> []
         -- |
         -- Test a predicate by substituting all type vars with associated
         -- tested types.
         analyzePredicate :: Pred -> Q Bool
         analyzePredicate = \case
-#if MIN_VERSION_template_haskell(2,10,0)
           AppT (AppT EqualityT _) _ -> return True
-          AppT (ConT n) t -> do
-            let t' = replaceTypeVars actualTypeByVarName t
-            isProperInstance n [t']
+          AppT (ConT n) t ->
+            case replaceTypeVars actualTypeByVarName t of
+              Just t' -> isProperInstance n [t']
+              _ -> return False
           _ -> return True
-#else
-          EqualP _ _ -> return True
-          ClassP n tl -> do
-            let tl' = map (replaceTypeVars actualTypeByVarName) tl
-            isProperInstance n tl'
-#endif
 
 unapplyType :: Type -> [Type]
 unapplyType = \case
@@ -101,9 +91,9 @@
   t -> [t]
 
 -- | Deeply traverse the type signature and replace all vars in it.
-replaceTypeVars :: (Name -> Type) -> Type -> Type
+replaceTypeVars :: (Name -> Maybe Type) -> Type -> Maybe Type
 replaceTypeVars f = \case
-  AppT l r -> AppT (replaceTypeVars f l) (replaceTypeVars f r)
+  AppT l r -> AppT <$> replaceTypeVars f l <*> replaceTypeVars f r
   VarT n -> f n
-  t -> t
+  t -> Just t
 
diff --git a/test/Main.hs b/test/Main.hs
--- a/test/Main.hs
+++ b/test/Main.hs
@@ -14,6 +14,11 @@
 
 
 main = defaultMain $ testGroup "" $ [
+    testCase "Issue #6" $ do
+      assertBool "" $ read $( do
+          r <- isProperInstance ''Monoid [VarT (mkName "v")]
+          stringE $ show $ r
+        ),
     testCase "existingInstance" $ do
       assertBool "" $ read $( do
           t <- [t| (Int, Int) |]
diff --git a/th-instance-reification.cabal b/th-instance-reification.cabal
--- a/th-instance-reification.cabal
+++ b/th-instance-reification.cabal
@@ -1,5 +1,5 @@
 name: th-instance-reification
-version: 0.1.5
+version: 0.1.5.1
 synopsis: Fixed versions of instances reification functions
 description:
   Provides fixed versions of 'reifyInstances' and 'isInstance' as per