diff --git a/src/emu/mux.c b/src/emu/mux.c
index 2da80db..db015cf 100644
--- a/src/emu/mux.c
+++ b/src/emu/mux.c
@@ -97,13 +97,15 @@ cb_select(struct chan *sel_chan, void *ptr)
 				value_str(sel_value), input->chan->name);
 	}
 
-	/* Set to null by default */
-	struct value out_value = value_null();
+	struct value out_value = mux->def;
 	if (input != NULL && chan_read(input->chan, &out_value) != 0) {
 		err("chan_read() failed");
 		return -1;
 	}
 
+	dbg("setting output chan %s to %s",
+			mux->output->name, value_str(out_value, buf));
+
 	if (chan_set(mux->output, out_value) != 0) {
 		err("chan_set() failed");
 		return -1;
@@ -180,6 +182,7 @@ mux_init(struct mux *mux,
 	mux->output = output;
 	mux->ninputs = ninputs;
 	mux->inputs = calloc(ninputs, sizeof(struct mux_input));
+	mux->def = value_null();
 
 	if (mux->inputs == NULL) {
 		err("calloc failed:");
@@ -245,3 +248,9 @@ mux_set_input(struct mux *mux, int64_t index, struct chan *chan)
 
 	return 0;
 }
+
+void
+mux_set_default(struct mux *mux, struct value def)
+{
+	mux->def = def;
+}
diff --git a/src/emu/mux.h b/src/emu/mux.h
index d9735a7..b023356 100644
--- a/src/emu/mux.h
+++ b/src/emu/mux.h
@@ -31,6 +31,7 @@ struct mux {
 	mux_select_func_t select_func;
 	struct chan *select;
 	struct chan *output;
+	struct value def;
 };
 
 void mux_input_init(struct mux_input *mux,
@@ -54,4 +55,6 @@ USE_RET struct mux_input *mux_get_input(struct mux *mux,
 USE_RET int mux_register(struct mux *mux,
 		struct bay *bay);
 
+void mux_set_default(struct mux *mux, struct value def);
+
 #endif /* MUX_H */