diff --git a/src/contracts.coffee b/src/contracts.coffee
index ce951fb..7ed1d7d 100644
--- a/src/contracts.coffee
+++ b/src/contracts.coffee
@@ -256,7 +256,8 @@ fun = (dom, rng, options) ->
         true
       else
         if prevWasOpt
-          throw new Error "Illagal arguments: required argument following an optional argument."
+          throw new Error "Illagal arguments: required argument following an" +
+            "optional argument. #{curr}"
         else
           false
     ), false