diff --git a/src/comp_helper.h b/src/comp_helper.h
index c17aaa76..8292fc8d 100644
--- a/src/comp_helper.h
+++ b/src/comp_helper.h
@@ -39,6 +39,6 @@ json_t* dump_headers(const nghttp2_nv *nva, size_t nvlen);
 
 void output_json_header(int side);
 
-void output_json_footer();
+void output_json_footer(void);
 
 #endif /* NGHTTP2_COMP_HELPER_H */