diff --git a/multi_button.h b/multi_button.h
index b066ade..c468458 100644
--- a/multi_button.h
+++ b/multi_button.h
@@ -48,7 +48,7 @@ typedef struct Button {
 extern "C" {
 #endif
 
-void button_init(struct Button* handle, uint8_t(*pin_level)(), uint8_t active_level, uint8_t button_id);
+void button_init(struct Button* handle, uint8_t(*pin_level)(uint8_t), uint8_t active_level, uint8_t button_id);
 void button_attach(struct Button* handle, PressEvent event, BtnCallback cb);
 PressEvent get_button_event(struct Button* handle);
 int  button_start(struct Button* handle);