diff --git a/src/Math/Haskellator/Internal/AstProcessingSteps/Evaluate.hs b/src/Math/Haskellator/Internal/AstProcessingSteps/Evaluate.hs
index e8f1640..9820c71 100644
--- a/src/Math/Haskellator/Internal/AstProcessingSteps/Evaluate.hs
+++ b/src/Math/Haskellator/Internal/AstProcessingSteps/Evaluate.hs
@@ -38,9 +38,9 @@ execVal = return
 
 execBinOp :: Value Dimension -> Op -> Value Dimension -> SimpleAstFold (Value Dimension)
 execBinOp lhs Plus  rhs | unit lhs =~= unit rhs = return $ combineValues (+) lhs rhs
-                        | otherwise = throwError $ Error RuntimeError $ "Cannot add units " ++ show lhs ++ " and " ++ show rhs
+                        | otherwise = throwError $ Error RuntimeError $ "Cannot add units " ++ show (unit lhs) ++ " and " ++ show (unit rhs)
 execBinOp lhs Minus rhs | unit lhs =~= unit rhs = return $ combineValues (-) lhs rhs
-                        | otherwise = throwError $ Error RuntimeError $ "Cannot subtract units " ++ show lhs ++ " and " ++ show rhs
+                        | otherwise = throwError $ Error RuntimeError $ "Cannot subtract units " ++ show (unit lhs) ++ " and " ++ show (unit rhs)
 execBinOp lhs Mult  rhs = do
     let u = mergeUnits (unit lhs) (unit rhs)
     return $ Value (value lhs * value rhs) u
diff --git a/src/Math/Haskellator/Internal/TH/UnitGeneration.hs b/src/Math/Haskellator/Internal/TH/UnitGeneration.hs
index a7b404d..01591a8 100644
--- a/src/Math/Haskellator/Internal/TH/UnitGeneration.hs
+++ b/src/Math/Haskellator/Internal/TH/UnitGeneration.hs
@@ -129,6 +129,7 @@ genUnitExp = [d|
     show (UnitExp u i) = show u ++ "^" ++ show i
 
   instance Eq UnitExp where
+    (UnitExp $(return $ ConP (mkName "Multiplier") [] []) _) == (UnitExp $(return $ ConP (mkName "Multiplier") [] []) _) = True
     (UnitExp u1 i1) == (UnitExp u2 i2) = u1 == u2 && i1 == i2
   |]