diff --git a/src/main/java/org/elasticsearch/index/mapper/DocumentMapper.java b/src/main/java/org/elasticsearch/index/mapper/DocumentMapper.java
index fa459d36b13..01a70b5fe1c 100644
--- a/src/main/java/org/elasticsearch/index/mapper/DocumentMapper.java
+++ b/src/main/java/org/elasticsearch/index/mapper/DocumentMapper.java
@@ -585,7 +585,9 @@ public class DocumentMapper implements ToXContent {
         public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
             builder.startObject();
             builder.field("script", script);
-            builder.field("lang", language);
+            if (language != null) {
+                builder.field("lang", language);
+            }
             if (parameters != null) {
                 builder.field("params", parameters);
             }
diff --git a/src/main/java/org/elasticsearch/script/ScriptParameterParser.java b/src/main/java/org/elasticsearch/script/ScriptParameterParser.java
index 95d60a58a9b..8addcd6ebc7 100644
--- a/src/main/java/org/elasticsearch/script/ScriptParameterParser.java
+++ b/src/main/java/org/elasticsearch/script/ScriptParameterParser.java
@@ -26,8 +26,13 @@ import org.elasticsearch.common.xcontent.XContentParser;
 import org.elasticsearch.script.ScriptService.ScriptType;
 
 import java.io.IOException;
-import java.util.*;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.Map;
 import java.util.Map.Entry;
+import java.util.Set;
 
 public class ScriptParameterParser {
 
@@ -102,12 +107,12 @@ public class ScriptParameterParser {
             String parameterName = entry.getKey();
             Object parameterValue = entry.getValue();
             if (ScriptService.SCRIPT_LANG.match(parameterName)) {
-               if (parameterValue instanceof String) {
+                if (parameterValue instanceof String || parameterValue == null) {
                    lang = (String) parameterValue;
                    if (removeMatchedEntries) {
                        itr.remove();
                    }
-               } else {
+                } else {
                    throw new ScriptParameterParseException("Value must be of type String: [" + parameterName + "]");
                }
             } else {
diff --git a/src/test/java/org/elasticsearch/bwcompat/ScriptTransformBackwardsCompatibilityTests.java b/src/test/java/org/elasticsearch/bwcompat/ScriptTransformBackwardsCompatibilityTests.java
new file mode 100644
index 00000000000..7d8a26812aa
--- /dev/null
+++ b/src/test/java/org/elasticsearch/bwcompat/ScriptTransformBackwardsCompatibilityTests.java
@@ -0,0 +1,89 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.bwcompat;
+
+import com.google.common.collect.ImmutableMap;
+
+import org.elasticsearch.action.get.GetResponse;
+import org.elasticsearch.common.xcontent.XContentBuilder;
+import org.elasticsearch.common.xcontent.XContentFactory;
+import org.elasticsearch.test.ElasticsearchBackwardsCompatIntegrationTest;
+import org.junit.Test;
+
+import java.io.IOException;
+
+import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertAcked;
+import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertExists;
+import static org.hamcrest.Matchers.both;
+import static org.hamcrest.Matchers.hasEntry;
+import static org.hamcrest.Matchers.hasKey;
+import static org.hamcrest.Matchers.not;
+
+public class ScriptTransformBackwardsCompatibilityTests extends ElasticsearchBackwardsCompatIntegrationTest {
+
+    @Test
+    public void testTransformWithNoLangSpecified() throws Exception {
+        XContentBuilder builder = XContentFactory.jsonBuilder().startObject();
+        builder.field("transform");
+        if (getRandom().nextBoolean()) {
+            // Single transform
+            builder.startObject();
+            buildTransformScript(builder);
+            builder.endObject();
+        } else {
+            // Multiple transforms
+            int total = between(1, 10);
+            int actual = between(0, total - 1);
+            builder.startArray();
+            for (int s = 0; s < total; s++) {
+                builder.startObject();
+                if (s == actual) {
+                    buildTransformScript(builder);
+                } else {
+                    builder.field("script", "true");
+                }
+                builder.endObject();
+            }
+            builder.endArray();
+        }
+        assertAcked(client().admin().indices().prepareCreate("test").addMapping("test", builder));
+
+        indexRandom(getRandom().nextBoolean(), client().prepareIndex("test", "test", "notitle").setSource("content", "findme"), client()
+                .prepareIndex("test", "test", "badtitle").setSource("content", "findme", "title", "cat"),
+                client().prepareIndex("test", "test", "righttitle").setSource("content", "findme", "title", "table"));
+        GetResponse response = client().prepareGet("test", "test", "righttitle").get();
+        assertExists(response);
+        assertThat(response.getSource(), both(hasEntry("content", (Object) "findme")).and(not(hasKey("destination"))));
+
+        response = client().prepareGet("test", "test", "righttitle").setTransformSource(true).get();
+        assertExists(response);
+        assertThat(response.getSource(), both(hasEntry("destination", (Object) "findme")).and(not(hasKey("content"))));
+    }
+
+    private void buildTransformScript(XContentBuilder builder) throws IOException {
+        String script = "if (ctx._source['title']?.startsWith('t')) { ctx._source['destination'] = ctx._source[sourceField] }; ctx._source.remove(sourceField);";
+        if (getRandom().nextBoolean()) {
+            script = script.replace("sourceField", "'content'");
+        } else {
+            builder.field("params", ImmutableMap.of("sourceField", "content"));
+        }
+        builder.field("script", script);
+    }
+}
diff --git a/src/test/java/org/elasticsearch/index/mapper/TransformOnIndexMapperIntegrationTest.java b/src/test/java/org/elasticsearch/index/mapper/TransformOnIndexMapperIntegrationTest.java
index 6b2180c840e..443ce1b39fb 100644
--- a/src/test/java/org/elasticsearch/index/mapper/TransformOnIndexMapperIntegrationTest.java
+++ b/src/test/java/org/elasticsearch/index/mapper/TransformOnIndexMapperIntegrationTest.java
@@ -130,7 +130,7 @@ public class TransformOnIndexMapperIntegrationTest extends ElasticsearchIntegrat
             // Single transform
             builder.startObject();
             buildTransformScript(builder);
-            builder.field("lang", GroovyScriptEngineService.NAME);
+            builder.field("lang", randomFrom(null, GroovyScriptEngineService.NAME));
             builder.endObject();
         } else {
             // Multiple transforms
@@ -144,7 +144,7 @@ public class TransformOnIndexMapperIntegrationTest extends ElasticsearchIntegrat
                 } else {
                     builder.field("script", "true");
                 }
-                builder.field("lang", GroovyScriptEngineService.NAME);
+                builder.field("lang", randomFrom(null, GroovyScriptEngineService.NAME));
                 builder.endObject();
             }
             builder.endArray();