diff --git a/externals/mmonoplayer/atari2600~.c b/externals/mmonoplayer/atari2600~.c
index a0aff2da06b3851df25220dfdf920b86ee3b197e..55bce8a8027313616afabdf4298857527b3df91f 100644
--- a/externals/mmonoplayer/atari2600~.c
+++ b/externals/mmonoplayer/atari2600~.c
@@ -122,7 +122,7 @@ void atari_2600_tilde_setup(void)
 {
 	atari_2600_class = class_new(gensym("atari_2600~"),
 			(t_newmethod)atari_2600_new, 0, sizeof(t_atari_2600), CLASS_DEFAULT, A_DEFFLOAT, 0);
-	class_addmethod(atari_2600_class, (t_method)atari_2600_dsp, gensym("dsp"), 0);
+	class_addmethod(atari_2600_class, (t_method)atari_2600_dsp, gensym("dsp"), A_CANT, 0);
 }
 
 void *atari_2600_new(void)