diff --git a/thread_sync.c b/thread_sync.c
index 7bdef32682..e7702f17d0 100644
--- a/thread_sync.c
+++ b/thread_sync.c
@@ -1308,6 +1308,7 @@ condvar_ptr(VALUE self)
 
     /* forked children can't reach into parent thread stacks */
     if (cv->fork_gen != fork_gen) {
+        cv->fork_gen = fork_gen;
         list_head_init(&cv->waitq);
     }