diff --git a/Colada/Features.hs b/Colada/Features.hs
--- a/Colada/Features.hs
+++ b/Colada/Features.hs
@@ -33,7 +33,7 @@
 at i (Zip _ rs) | i > 0  = index (i+1) rs
 at _ z = safeCursor z    -- i == 0
 
-index :: Num a => a -> [b] -> Maybe b
+index :: (Eq a, Num a) => a -> [b] -> Maybe b
 index 1 (x:_)  = Just x
 index _ []  = Nothing
 index i (_:xs) = index (i-1) xs
diff --git a/colada.cabal b/colada.cabal
--- a/colada.cabal
+++ b/colada.cabal
@@ -1,5 +1,5 @@
 Name:                colada
-Version:             0.4.1
+Version:             0.4.2
 Synopsis:            Colada implements incremental word class class induction 
                      using online LDA
 Description:  Colada implements incremental word class class induction using