diff --git a/thread_win32.ci b/thread_win32.ci
index d84cd341cc..9d151cdb6d 100644
--- a/thread_win32.ci
+++ b/thread_win32.ci
@@ -423,6 +423,8 @@ thread_start_func_1(void *th_ptr)
     return 0;
 }
 
+extern size_t rb_gc_stack_maxsize;
+
 static int
 native_thread_create(rb_thread_t *th)
 {