1 files changed, 1 insertions, 0 deletions
diff --git a/io.c b/io.c
index fdea5d2722..f0d314396b 100644
--- a/io.c
+++ b/io.c
@@ -3301,6 +3301,7 @@ pipe_finalize(rb_io_t *fptr, int noraise)
 void
 rb_io_synchronized(rb_io_t *fptr)
 {
+    rb_io_check_initialized(fptr);
     fptr->mode |= FMODE_SYNC;
 }