diff --git a/thread.c b/thread.c
index f50ad3de6a..acb53354fd 100644
--- a/thread.c
+++ b/thread.c
@@ -194,7 +194,7 @@ vm_check_ints_blocking(rb_execution_context_t *ec)
 static int
 vm_living_thread_num(rb_vm_t *vm)
 {
-    return (int)vm->living_thread_num;
+    return vm->living_thread_num;
 }
 
 #if THREAD_DEBUG
diff --git a/vm_core.h b/vm_core.h
index 176ec9d333..8bce7892b5 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -526,8 +526,8 @@ typedef struct rb_vm_struct {
 
     struct list_head waiting_fds; /* <=> struct waiting_fd */
     struct list_head living_threads;
-    size_t living_thread_num;
     VALUE thgroup_default;
+    int living_thread_num;
 
     unsigned int running: 1;
     unsigned int thread_abort_on_exception: 1;