1 files changed, 1 insertions, 0 deletions
diff --git a/vm.c b/vm.c
index 5778cfe42a..5cc4658290 100644
--- a/vm.c
+++ b/vm.c
@@ -1551,6 +1551,7 @@ rb_thread_mark(void *ptr)
 	RUBY_MARK_UNLESS_NULL(th->last_status);
 
 	RUBY_MARK_UNLESS_NULL(th->locking_mutex);
+	RUBY_MARK_UNLESS_NULL(th->keeping_mutexes);
 
 	rb_mark_tbl(th->local_storage);