diff --git a/thread.c b/thread.c
index 7034c21f29..56f1399d41 100644
--- a/thread.c
+++ b/thread.c
@@ -1771,7 +1771,7 @@ VALUE
 rb_thread_io_blocking_call(rb_blocking_function_t *func, void *data1, int fd, int events)
 {
     rb_execution_context_t *volatile ec = GET_EC();
-    rb_thread_t *th = rb_ec_thread_ptr(ec);
+    rb_thread_t *volatile th = rb_ec_thread_ptr(ec);
 
     RUBY_DEBUG_LOG("th:%u fd:%d ev:%d", rb_th_serial(th), fd, events);