diff --git a/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/MlConfigMigrator.java b/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/MlConfigMigrator.java
index bb3735f8aa3f1..fbf8c3c804eef 100644
--- a/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/MlConfigMigrator.java
+++ b/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/MlConfigMigrator.java
@@ -402,7 +402,8 @@ private void createConfigIndex(ActionListener<Boolean> listener) {
     public static Job updateJobForMigration(Job job) {
         Job.Builder builder = new Job.Builder(job);
         Map<String, Object> custom = job.getCustomSettings() == null ? new HashMap<>() : new HashMap<>(job.getCustomSettings());
-        custom.put(MIGRATED_FROM_VERSION, job.getJobVersion());
+        String version = job.getJobVersion() != null ? job.getJobVersion().toString() : null;
+        custom.put(MIGRATED_FROM_VERSION, version);
         builder.setCustomSettings(custom);
         // Increase the model memory limit for 6.1 - 6.3 jobs
         Version jobVersion = job.getJobVersion();
diff --git a/x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/MlConfigMigratorTests.java b/x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/MlConfigMigratorTests.java
index d9ea035e58234..cff299b9fa1aa 100644
--- a/x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/MlConfigMigratorTests.java
+++ b/x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/MlConfigMigratorTests.java
@@ -11,6 +11,7 @@
 import org.elasticsearch.cluster.ClusterName;
 import org.elasticsearch.cluster.ClusterState;
 import org.elasticsearch.cluster.metadata.MetaData;
+import org.elasticsearch.common.io.stream.NamedWriteableRegistry;
 import org.elasticsearch.persistent.PersistentTasksCustomMetaData;
 import org.elasticsearch.test.ESTestCase;
 import org.elasticsearch.xpack.core.ml.MlMetadata;
@@ -21,9 +22,11 @@
 import org.elasticsearch.xpack.core.ml.job.config.Job;
 import org.elasticsearch.xpack.core.ml.job.config.JobTests;
 
+import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
+import java.util.Date;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -125,7 +128,7 @@ public void testUpdateJobForMigration() {
         Job migratedJob = MlConfigMigrator.updateJobForMigration(oldJob.build());
         assertEquals(Version.CURRENT, migratedJob.getJobVersion());
         assertTrue(migratedJob.getCustomSettings().containsKey(MlConfigMigrator.MIGRATED_FROM_VERSION));
-        assertEquals(oldVersion, migratedJob.getCustomSettings().get(MlConfigMigrator.MIGRATED_FROM_VERSION));
+        assertEquals(oldVersion.toString(), migratedJob.getCustomSettings().get(MlConfigMigrator.MIGRATED_FROM_VERSION));
     }
 
     public void testUpdateJobForMigration_GivenV54Job() {
@@ -138,6 +141,12 @@ public void testUpdateJobForMigration_GivenV54Job() {
         assertTrue(migratedJob.getCustomSettings().containsKey(MlConfigMigrator.MIGRATED_FROM_VERSION));
     }
 
+    public void testSerialisationOfUpdatedJob() throws IOException {
+        Job migratedJob = MlConfigMigrator.updateJobForMigration(JobTests.buildJobBuilder("pre-migration").build(new Date()));
+        Job copy = copyWriteable(migratedJob, new NamedWriteableRegistry(Collections.emptyList()), Job::new, Version.CURRENT);
+        assertEquals(migratedJob, copy);
+    }
+
     public void testFilterFailedJobConfigWrites() {
         List<Job> jobs = new ArrayList<>();
         jobs.add(JobTests.buildJobBuilder("foo").build());