diff --git a/lib/Quox/NatExtra.idr b/lib/Quox/NatExtra.idr
index 64248ec..714add1 100644
--- a/lib/Quox/NatExtra.idr
+++ b/lib/Quox/NatExtra.idr
@@ -82,7 +82,7 @@ namespace Int
 
   export %inline
   fromHex : String -> Maybe Int
-  fromHex = fromHex' 0
+  fromHex str = do guard $ str /= ""; fromHex' 0 str
 
 namespace Nat
   export