diff --git a/vm_core.h b/vm_core.h
index 1ab61e10f4..4d4ed2387d 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -1078,7 +1078,7 @@ typedef struct rb_thread_struct {
 static inline unsigned int
 rb_th_serial(const rb_thread_t *th)
 {
-    return (unsigned int)th->serial;
+    return th ? (unsigned int)th->serial : 0;
 }
 
 typedef enum {