diff --git a/src/bindings.cc b/src/bindings.cc
index df1de3ec..5ae389b1 100644
--- a/src/bindings.cc
+++ b/src/bindings.cc
@@ -481,6 +481,7 @@ bool Bindings::addButton(const std::string &but, MouseContext::MC context,
                            MouseAction::MA action, PyObject *callback)
 {
   assert(context >= 0 && context < MouseContext::NUM_MOUSE_CONTEXT);
+  assert(action >= 0 && context < MouseContext::NUM_MOUSE_ACTION);
   
   Binding b(0,0);
   if (!translate(but, b, false))