diff --git a/src/ep_devctxt.c b/src/ep_devctxt.c
index b906ab5..33d6913 100755
--- a/src/ep_devctxt.c
+++ b/src/ep_devctxt.c
@@ -172,6 +172,9 @@ static PyObject *devctxt_select_bitmap(DevCtxtObject *self, BitmapObject *newbmp
 	}
   }
   Log(LDEBUG, "replacement sequence");
+  ASSERT(old_bitmap);
+  ASSERT(self->selected_bitmap);
+  ASSERT(newbmp);
   Py_DECREF(self->selected_bitmap);
   self->selected_bitmap = newbmp;
   Py_INCREF(self->selected_bitmap);