diff --git a/source/common/upstream/cluster_manager_impl.cc b/source/common/upstream/cluster_manager_impl.cc
index 81160b941eb0..b3f89cb6213c 100644
--- a/source/common/upstream/cluster_manager_impl.cc
+++ b/source/common/upstream/cluster_manager_impl.cc
@@ -95,7 +95,12 @@ void ClusterManagerInitHelper::maybeFinishInitialize() {
     if (!started_secondary_initialize_) {
       log().info("cm init: initializing secondary clusters");
       started_secondary_initialize_ = true;
-      for (Cluster* cluster : secondary_init_clusters_) {
+      // Cluster::initialize() method can modify the list of secondary_init_clusters_ to remove
+      // the item currently being initialized, so we eschew range-based-for and do this complicated
+      // dance to increment the iterator before calling initialize.
+      for (auto iter = secondary_init_clusters_.begin(); iter != secondary_init_clusters_.end();) {
+        Cluster* cluster = *iter;
+        ++iter;
         cluster->initialize();
       }
     }
diff --git a/test/common/upstream/cluster_manager_impl_test.cc b/test/common/upstream/cluster_manager_impl_test.cc
index a48e67f374ab..53a274bdcc03 100644
--- a/test/common/upstream/cluster_manager_impl_test.cc
+++ b/test/common/upstream/cluster_manager_impl_test.cc
@@ -838,4 +838,25 @@ TEST(ClusterManagerInitHelper, AddSecondaryAfterSecondaryInit) {
   cluster2.initialize_callback_();
 }
 
+TEST(ClusterManagerInitHelper, RemoveClusterWithinInitLoop) {
+  // Tests the scenario encountered in Issue 903: The cluster was removed from
+  // the secondary init list while traversing the list.
+
+  InSequence s;
+  ClusterManagerInitHelper init_helper;
+  NiceMock<MockCluster> cluster;
+  ON_CALL(cluster, initializePhase()).WillByDefault(Return(Cluster::InitializePhase::Secondary));
+  init_helper.addCluster(cluster);
+
+  // Set up the scenario seen in Issue 903 where initialize() ultimately results
+  // in the removeCluster() call. In the real bug this was a long and complex call
+  // chain.
+  EXPECT_CALL(cluster, initialize())
+      .WillOnce(Invoke([&]() -> void { init_helper.removeCluster(cluster); }));
+
+  // Now call onStaticLoadComplete which will exercise maybeFinishInitialize()
+  // which calls initialize() on the members of the secondary init list.
+  init_helper.onStaticLoadComplete();
+}
+
 } // Upstream