diff --git a/core/src/main/java/org/elasticsearch/action/resync/TransportResyncReplicationAction.java b/core/src/main/java/org/elasticsearch/action/resync/TransportResyncReplicationAction.java
index c723a175ad7..6b84522d23a 100644
--- a/core/src/main/java/org/elasticsearch/action/resync/TransportResyncReplicationAction.java
+++ b/core/src/main/java/org/elasticsearch/action/resync/TransportResyncReplicationAction.java
@@ -93,7 +93,7 @@ public class TransportResyncReplicationAction extends TransportWriteAction<Resyn
         if (node.getVersion().onOrAfter(Version.V_6_0_0_alpha1)) {
             super.sendReplicaRequest(replicaRequest, node, listener);
         } else {
-            listener.onResponse(new ReplicaResponse(SequenceNumbersService.UNASSIGNED_SEQ_NO));
+            listener.onResponse(new ReplicaResponse(SequenceNumbersService.PRE_60_NODE_LOCAL_CHECKPOINT));
         }
     }
 
diff --git a/core/src/main/java/org/elasticsearch/action/support/replication/TransportReplicationAction.java b/core/src/main/java/org/elasticsearch/action/support/replication/TransportReplicationAction.java
index e885225cb80..ae245fe252c 100644
--- a/core/src/main/java/org/elasticsearch/action/support/replication/TransportReplicationAction.java
+++ b/core/src/main/java/org/elasticsearch/action/support/replication/TransportReplicationAction.java
@@ -1023,6 +1023,12 @@ public abstract class TransportReplicationAction<
         }
 
         public ReplicaResponse(long localCheckpoint) {
+            /*
+             * A replica should always know its own local checkpoint so this should always be a valid sequence number or the pre-6.0 local
+             * checkpoint value when simulating responses to replication actions that pre-6.0 nodes are not aware of (e.g., the global
+             * checkpoint background sync, and the primary/replica resync).
+             */
+            assert localCheckpoint != SequenceNumbersService.UNASSIGNED_SEQ_NO;
             this.localCheckpoint = localCheckpoint;
         }
 
diff --git a/core/src/main/java/org/elasticsearch/index/seqno/GlobalCheckpointSyncAction.java b/core/src/main/java/org/elasticsearch/index/seqno/GlobalCheckpointSyncAction.java
index 897dc9beb32..c7059d05919 100644
--- a/core/src/main/java/org/elasticsearch/index/seqno/GlobalCheckpointSyncAction.java
+++ b/core/src/main/java/org/elasticsearch/index/seqno/GlobalCheckpointSyncAction.java
@@ -89,7 +89,7 @@ public class GlobalCheckpointSyncAction extends TransportReplicationAction<
         if (node.getVersion().onOrAfter(Version.V_6_0_0_alpha1)) {
             super.sendReplicaRequest(replicaRequest, node, listener);
         } else {
-            listener.onResponse(new ReplicaResponse(SequenceNumbersService.UNASSIGNED_SEQ_NO));
+            listener.onResponse(new ReplicaResponse(SequenceNumbersService.PRE_60_NODE_LOCAL_CHECKPOINT));
         }
     }
 
diff --git a/core/src/main/java/org/elasticsearch/index/seqno/GlobalCheckpointTracker.java b/core/src/main/java/org/elasticsearch/index/seqno/GlobalCheckpointTracker.java
index c112e82998b..447815cf9af 100644
--- a/core/src/main/java/org/elasticsearch/index/seqno/GlobalCheckpointTracker.java
+++ b/core/src/main/java/org/elasticsearch/index/seqno/GlobalCheckpointTracker.java
@@ -450,6 +450,9 @@ public class GlobalCheckpointTracker extends AbstractIndexShardComponent {
         assert lcps.localCheckpoint != SequenceNumbersService.PRE_60_NODE_LOCAL_CHECKPOINT ||
             localCheckpoint == SequenceNumbersService.PRE_60_NODE_LOCAL_CHECKPOINT :
             "pre-6.0 shard copy " + allocationId + " unexpected to send valid local checkpoint " + localCheckpoint;
+        // a local checkpoint for a shard copy should be a valid sequence number or the pre-6.0 sequence number indicator
+        assert localCheckpoint != SequenceNumbersService.UNASSIGNED_SEQ_NO :
+                "invalid local checkpoint for shard copy [" + allocationId + "]";
         if (localCheckpoint > lcps.localCheckpoint) {
             logger.trace("updated local checkpoint of [{}] from [{}] to [{}]", allocationId, lcps.localCheckpoint, localCheckpoint);
             lcps.localCheckpoint = localCheckpoint;
diff --git a/core/src/test/java/org/elasticsearch/index/seqno/GlobalCheckpointTrackerTests.java b/core/src/test/java/org/elasticsearch/index/seqno/GlobalCheckpointTrackerTests.java
index f06ffc0d45f..2f7d2dd15ce 100644
--- a/core/src/test/java/org/elasticsearch/index/seqno/GlobalCheckpointTrackerTests.java
+++ b/core/src/test/java/org/elasticsearch/index/seqno/GlobalCheckpointTrackerTests.java
@@ -737,7 +737,7 @@ public class GlobalCheckpointTrackerTests extends ESTestCase {
     private static void randomLocalCheckpointUpdate(GlobalCheckpointTracker gcp) {
         String allocationId = randomFrom(gcp.localCheckpoints.keySet());
         long currentLocalCheckpoint = gcp.localCheckpoints.get(allocationId).getLocalCheckpoint();
-        gcp.updateLocalCheckpoint(allocationId, currentLocalCheckpoint + randomInt(5));
+        gcp.updateLocalCheckpoint(allocationId, Math.max(SequenceNumbersService.NO_OPS_PERFORMED, currentLocalCheckpoint + randomInt(5)));
     }
 
     private static void randomMarkInSync(GlobalCheckpointTracker gcp) {