diff --git a/namespace.c b/namespace.c
index 855aa375aa..25b960f0f6 100644
--- a/namespace.c
+++ b/namespace.c
@@ -52,7 +52,7 @@ VALUE rb_resolve_feature_path(VALUE klass, VALUE fname);
 static VALUE rb_namespace_inspect(VALUE obj);
 
 int
-rb_namespace_available()
+rb_namespace_available(void)
 {
     const char *env;
     if (namespace_availability) {