diff --git a/debug.c b/debug.c
index 3a6caca58a..6c9d6563db 100644
--- a/debug.c
+++ b/debug.c
@@ -474,7 +474,7 @@ ruby_debug_log(const char *file, int line, const char *func_name, const char *fm
         // thread information
         const rb_thread_t *th = GET_THREAD();
         if (r && len < MAX_DEBUG_LOG_MESSAGE_LEN) {
-            r = snprintf(buff + len, MAX_DEBUG_LOG_MESSAGE_LEN - len, "\tth:%u", (unsigned int)th->serial);
+            r = snprintf(buff + len, MAX_DEBUG_LOG_MESSAGE_LEN - len, "\tth:%u", rb_th_serial(th));
             if (r < 0) rb_bug("ruby_debug_log returns %d\n", r);
             len += r;
         }
diff --git a/vm_core.h b/vm_core.h
index 4de0777292..58fb1e7e3b 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -1078,6 +1078,12 @@ typedef struct rb_thread_struct {
 #endif
 } rb_thread_t;
 
+static inline unsigned int
+rb_th_serial(const rb_thread_t *th)
+{
+    return (unsigned int)th->serial;
+}
+
 typedef enum {
     VM_DEFINECLASS_TYPE_CLASS           = 0x00,
     VM_DEFINECLASS_TYPE_SINGLETON_CLASS = 0x01,