diff --git a/lib/sippy_cup/media/dtmf_payload.rb b/lib/sippy_cup/media/dtmf_payload.rb
index de62bdf..7b27bd0 100644
--- a/lib/sippy_cup/media/dtmf_payload.rb
+++ b/lib/sippy_cup/media/dtmf_payload.rb
@@ -24,7 +24,7 @@ def end_of_event=(bool)
         if bool
           @flags |= END_OF_EVENT
         else
-          @flags &= (0xf - END_OF_EVENT)
+          @flags &= ~END_OF_EVENT
         end
       end