diff --git a/quiche/quic/core/quic_flags_list.h b/quiche/quic/core/quic_flags_list.h
index e2470d5..d9fe60d 100644
--- a/quiche/quic/core/quic_flags_list.h
+++ b/quiche/quic/core/quic_flags_list.h
@@ -66,7 +66,7 @@
 // If true, do not bundle ACK while sending PATH_CHALLENGE on alternative path.
 QUIC_FLAG(quic_reloadable_flag_quic_not_bundle_ack_on_alternative_path, true)
 // If true, do not issue a new connection ID that has been claimed by another connection.
-QUIC_FLAG(quic_reloadable_flag_quic_check_cid_collision_when_issue_new_cid, false)
+QUIC_FLAG(quic_reloadable_flag_quic_check_cid_collision_when_issue_new_cid, true)
 // If true, enable server retransmittable on wire PING.
 QUIC_FLAG(quic_reloadable_flag_quic_enable_server_on_wire_ping, true)
 // If true, flush pending frames as well as pending padding bytes on connection migration.