diff --git a/src/main/java/org/elasticsearch/index/mapper/internal/TimestampFieldMapper.java b/src/main/java/org/elasticsearch/index/mapper/internal/TimestampFieldMapper.java
index aae21042878..cb828e2d994 100644
--- a/src/main/java/org/elasticsearch/index/mapper/internal/TimestampFieldMapper.java
+++ b/src/main/java/org/elasticsearch/index/mapper/internal/TimestampFieldMapper.java
@@ -305,7 +305,14 @@ public class TimestampFieldMapper extends DateFieldMapper implements InternalMap
                 this.enabledState = timestampFieldMapperMergeWith.enabledState;
             }
         } else {
-            if (!timestampFieldMapperMergeWith.defaultTimestamp().equals(defaultTimestamp)) {
+            if (timestampFieldMapperMergeWith.defaultTimestamp() == null && defaultTimestamp == null) {
+                return;
+            }
+            if (defaultTimestamp == null) {
+                mergeContext.addConflict("Cannot update default in _timestamp value. Value is null now encountering " + timestampFieldMapperMergeWith.defaultTimestamp());
+            } else if (timestampFieldMapperMergeWith.defaultTimestamp() == null) {
+                mergeContext.addConflict("Cannot update default in _timestamp value. Value is \" + defaultTimestamp.toString() + \" now encountering null");
+            } else if (!timestampFieldMapperMergeWith.defaultTimestamp().equals(defaultTimestamp)) {
                 mergeContext.addConflict("Cannot update default in _timestamp value. Value is " + defaultTimestamp.toString() + " now encountering " + timestampFieldMapperMergeWith.defaultTimestamp());
             }
             if (this.path != null) {
diff --git a/src/test/java/org/elasticsearch/index/mapper/timestamp/TimestampMappingTests.java b/src/test/java/org/elasticsearch/index/mapper/timestamp/TimestampMappingTests.java
index c15eb8c5cba..b6cda34ea51 100644
--- a/src/test/java/org/elasticsearch/index/mapper/timestamp/TimestampMappingTests.java
+++ b/src/test/java/org/elasticsearch/index/mapper/timestamp/TimestampMappingTests.java
@@ -22,6 +22,7 @@ package org.elasticsearch.index.mapper.timestamp;
 import org.apache.lucene.index.IndexOptions;
 import org.elasticsearch.Version;
 import org.elasticsearch.action.TimestampParsingException;
+import org.elasticsearch.action.admin.indices.delete.DeleteIndexRequest;
 import org.elasticsearch.action.index.IndexRequest;
 import org.elasticsearch.cluster.metadata.IndexMetaData;
 import org.elasticsearch.cluster.metadata.MappingMetaData;
@@ -541,6 +542,87 @@ public class TimestampMappingTests extends ElasticsearchSingleNodeTest {
         }
     }
 
+    /**
+     * Test case for #9204
+     */
+    @Test
+    public void testMergingNullValues() throws Exception {
+        // From trying to add another field with default = null
+        String mapping = XContentFactory.jsonBuilder().startObject()
+                .startObject("type")
+                .startObject("_timestamp")
+                    .field("enabled", true)
+                    .field("default", (String) null)
+                .endObject()
+                .endObject().endObject().string();
+        DocumentMapperParser parser = createIndex("test").mapperService().documentMapperParser();
+
+        DocumentMapper docMapper = parser.parse(mapping);
+        mapping = XContentFactory.jsonBuilder().startObject()
+                .startObject("type")
+                    .startObject("_timestamp")
+                        .field("enabled", true)
+                        .field("default", (String) null)
+                    .endObject()
+                    .startObject("properties")
+                        .startObject("foo")
+                            .field("type", "string")
+                        .endObject()
+                    .endObject()
+                .endObject().endObject().string();
+
+        DocumentMapper.MergeResult mergeResult = docMapper.merge(parser.parse(mapping), DocumentMapper.MergeFlags.mergeFlags().simulate(true));
+        assertThat(mergeResult.hasConflicts(), is(false));
+
+        client().admin().indices().delete(new DeleteIndexRequest("test")).get();
+
+        // From trying to update from null to non null
+        mapping = XContentFactory.jsonBuilder().startObject()
+                .startObject("type")
+                .startObject("_timestamp")
+                    .field("enabled", true)
+                    .field("default", (String) null)
+                .endObject()
+                .endObject().endObject().string();
+        parser = createIndex("test").mapperService().documentMapperParser();
+
+        docMapper = parser.parse(mapping);
+        mapping = XContentFactory.jsonBuilder().startObject()
+                .startObject("type")
+                    .startObject("_timestamp")
+                        .field("enabled", true)
+                        .field("default", "now")
+                    .endObject()
+                .endObject().endObject().string();
+
+        mergeResult = docMapper.merge(parser.parse(mapping), DocumentMapper.MergeFlags.mergeFlags().simulate(true));
+        assertThat(mergeResult.hasConflicts(), is(true));
+
+        client().admin().indices().delete(new DeleteIndexRequest("test")).get();
+
+        // From trying to update from non null to null
+        mapping = XContentFactory.jsonBuilder().startObject()
+                .startObject("type")
+                .startObject("_timestamp")
+                    .field("enabled", true)
+                    .field("default", "now")
+                .endObject()
+                .endObject().endObject().string();
+        parser = createIndex("test").mapperService().documentMapperParser();
+
+        docMapper = parser.parse(mapping);
+        mapping = XContentFactory.jsonBuilder().startObject()
+                .startObject("type")
+                    .startObject("_timestamp")
+                .field("enabled", true)
+                .field("default", (String) null)
+                    .endObject()
+                .endObject().endObject().string();
+
+        mergeResult = docMapper.merge(parser.parse(mapping), DocumentMapper.MergeFlags.mergeFlags().simulate(true));
+        assertThat(mergeResult.hasConflicts(), is(true));
+    }
+
     @Test
     public void testMergePaths() throws Exception {
         String[] possiblePathValues = {"some_path", "anotherPath", null};