--- a/pygtk/gtk/gdkevent.override	
+++ a/pygtk/gtk/gdkevent.override	
@@ -354,8 +354,9 @@ _wrap_gdk_event_tp_setattr(PyObject *self, char *attr, PyObject *value)
 	    char *tmp;
 	    STRING_CHECK();
 	    tmp = PyString_AsString(value);
-	    strncpy((char *) &event->client.data, tmp,
-                    sizeof(event->client.data));
+	    strncpy((char *) &event->client.data, tmp, 19);
+			if (strnlen(tmp) > 20)
+				event->client.data[20] = '\0';
 	    return 0;
 	}
 	break;