Added MESOS-9609 to the 1.9.1 CHANGELOG.
diff --git a/CHANGELOG b/CHANGELOG
index 8ec893b..f336fc9 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -220,6 +220,7 @@
 * This is a bug fix release.
 
 ** Bug
+  * [MESOS-9609] - Master check failure when marking agent unreachable.
   * [MESOS-9964] - Support destroying UCR containers in provisioning state.
   * [MESOS-9965] - Agent should not send `TASK_GONE_BY_OPERATOR` if the framework is not partition aware.
   * [MESOS-9966] - Agent crashes when trying to destroy orphaned nested container if root container is orphaned as well.