diff --git a/Text/Calculator.hs b/Text/Calculator.hs
--- a/Text/Calculator.hs
+++ b/Text/Calculator.hs
@@ -33,7 +33,7 @@
 
 -- | Only limited operators are available, 
 --   they are shown using some trick.
-instance (Num a) => Show (State1 a) where
+instance (Num a, Eq a) => Show (State1 a) where
    show f | 3 `f` 2 == 6 = "*"
           | 3 `f` 2 == 5 = "+"
           | 3 `f` 2 == 1 = "-"
diff --git a/wtk.cabal b/wtk.cabal
--- a/wtk.cabal
+++ b/wtk.cabal
@@ -1,5 +1,5 @@
 Name:		wtk
-Version:	0.2
+Version:	0.2.1
 License:	BSD3
 License-file: license
 Author:		Bartosz Wojcik