diff --git a/src/TypeUnary/Nat.hs b/src/TypeUnary/Nat.hs
--- a/src/TypeUnary/Nat.hs
+++ b/src/TypeUnary/Nat.hs
@@ -245,13 +245,13 @@
 
 -- | Index generation from integer. Can fail dynamically if the integer is
 -- too large.
-coerceToIndex :: (Show i, Num i, IsNat m) => i -> Index m
+coerceToIndex :: (Eq i, Show i, Num i, IsNat m) => i -> Index m
 coerceToIndex = coerceToIndex' nat
 
-coerceToIndex' :: (Show i, Num i) => Nat m -> i -> Index m
+coerceToIndex' :: (Eq i, Show i, Num i) => Nat m -> i -> Index m
 coerceToIndex' mOrig niOrig = loop mOrig niOrig
  where
-   loop :: (Show i, Num i) => Nat m -> i -> Index m
+   loop :: (Eq i, Show i, Num i) => Nat m -> i -> Index m
    loop Zero _        = error $ "coerceToIndex: out of bounds: "
                                 ++ show niOrig ++ " should be less than "
                                 ++ show mOrig
diff --git a/type-unary.cabal b/type-unary.cabal
--- a/type-unary.cabal
+++ b/type-unary.cabal
@@ -1,5 +1,5 @@
 Name:                type-unary
-Version:             0.2.4
+Version:             0.2.5
 Cabal-Version:       >= 1.6
 Synopsis:            
   Type-level and typed unary natural numbers, inequality proofs, vectors