diff --git a/src/state.c b/src/state.c
index 1b50eec34..4dafc964a 100644
--- a/src/state.c
+++ b/src/state.c
@@ -183,7 +183,7 @@ mrb_free_context(mrb_state *mrb, struct mrb_context *c)
   mrb_free(mrb, c);
 }
 
-int mrb_protect_atexit(mrb_state *mrb);
+void mrb_protect_atexit(mrb_state *mrb);
 
   MRB_API void
 mrb_close(mrb_state *mrb)