diff --git a/src/output_control.c b/src/output_control.c
index a9c41f053..47bb844d5 100644
--- a/src/output_control.c
+++ b/src/output_control.c
@@ -55,7 +55,7 @@ static void ao_command_async(struct audio_output *ao,
 	notify_signal(&ao->notify);
 }
 
-bool
+static bool
 audio_output_open(struct audio_output *ao,
 		  const struct audio_format *audio_format)
 {
diff --git a/src/output_control.h b/src/output_control.h
index dadda860d..fe3233e69 100644
--- a/src/output_control.h
+++ b/src/output_control.h
@@ -39,10 +39,6 @@ bool
 audio_output_init(struct audio_output *ao, const struct config_param *param,
 		  GError **error);
 
-bool
-audio_output_open(struct audio_output *ao,
-		  const struct audio_format *audio_format);
-
 /**
  * Opens or closes the device, depending on the "enabled" flag.
  *