diff --git a/lib/rubyonacid/factories/combination.rb b/lib/rubyonacid/factories/combination.rb
index <HASH>..<HASH> 100644
--- a/lib/rubyonacid/factories/combination.rb
+++ b/lib/rubyonacid/factories/combination.rb
@@ -69,6 +69,8 @@ class CombinationFactory < Factory
         end
       when WRAP
         return value % 1.0
+      else
+        raise "invalid constrain mode - must be CONSTRAIN or WRAP"
       end
     end
CombinationFactory now raises exception when invalid constant assigned.