diff --git a/core/src/main/java/org/elasticsearch/common/xcontent/XContentBuilder.java b/core/src/main/java/org/elasticsearch/common/xcontent/XContentBuilder.java
index af8e7534692..d26485a121d 100644
--- a/core/src/main/java/org/elasticsearch/common/xcontent/XContentBuilder.java
+++ b/core/src/main/java/org/elasticsearch/common/xcontent/XContentBuilder.java
@@ -53,7 +53,7 @@ import java.util.Map;
  */
 public final class XContentBuilder implements BytesStream, Releasable {
 
-    public static enum FieldCaseConversion {
+    public enum FieldCaseConversion {
         /**
          * No conversion will occur.
          */
@@ -251,14 +251,7 @@ public final class XContentBuilder implements BytesStream, Releasable {
     }
 
     public XContentBuilder field(XContentBuilderString name) throws IOException {
-        if (fieldCaseConversion == FieldCaseConversion.UNDERSCORE) {
-            generator.writeFieldName(name.underscore());
-        } else if (fieldCaseConversion == FieldCaseConversion.CAMELCASE) {
-            generator.writeFieldName(name.camelCase());
-        } else {
-            generator.writeFieldName(name.underscore());
-        }
-        return this;
+        return field(name, fieldCaseConversion);
     }
 
     public XContentBuilder field(XContentBuilderString name, FieldCaseConversion conversion) throws IOException {
@@ -273,22 +266,13 @@ public final class XContentBuilder implements BytesStream, Releasable {
     }
 
     public XContentBuilder field(String name) throws IOException {
-        if (fieldCaseConversion == FieldCaseConversion.UNDERSCORE) {
-            if (cachedStringBuilder == null) {
-                cachedStringBuilder = new StringBuilder();
-            }
-            name = Strings.toUnderscoreCase(name, cachedStringBuilder);
-        } else if (fieldCaseConversion == FieldCaseConversion.CAMELCASE) {
-            if (cachedStringBuilder == null) {
-                cachedStringBuilder = new StringBuilder();
-            }
-            name = Strings.toCamelCase(name, cachedStringBuilder);
-        }
-        generator.writeFieldName(name);
-        return this;
+        return field(name, fieldCaseConversion);
     }
 
     public XContentBuilder field(String name, FieldCaseConversion conversion) throws IOException {
+        if (name == null) {
+            throw new IllegalArgumentException("field name cannot be null");
+        }
         if (conversion == FieldCaseConversion.UNDERSCORE) {
             if (cachedStringBuilder == null) {
                 cachedStringBuilder = new StringBuilder();
diff --git a/core/src/test/java/org/elasticsearch/common/xcontent/builder/XContentBuilderTests.java b/core/src/test/java/org/elasticsearch/common/xcontent/builder/XContentBuilderTests.java
index 7ffafc004ab..9129e3c05b3 100644
--- a/core/src/test/java/org/elasticsearch/common/xcontent/builder/XContentBuilderTests.java
+++ b/core/src/test/java/org/elasticsearch/common/xcontent/builder/XContentBuilderTests.java
@@ -22,7 +22,6 @@ package org.elasticsearch.common.xcontent.builder;
 import org.apache.lucene.util.BytesRef;
 import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.geo.GeoPoint;
-import org.elasticsearch.common.io.FastCharArrayWriter;
 import org.elasticsearch.common.io.PathUtils;
 import org.elasticsearch.common.io.stream.BytesStreamOutput;
 import org.elasticsearch.common.xcontent.XContentBuilder;
@@ -39,6 +38,7 @@ import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Calendar;
+import java.util.Collections;
 import java.util.Date;
 import java.util.GregorianCalendar;
 import java.util.HashMap;
@@ -51,9 +51,6 @@ import static org.elasticsearch.common.xcontent.XContentBuilder.FieldCaseConvers
 import static org.elasticsearch.common.xcontent.XContentBuilder.FieldCaseConversion.UNDERSCORE;
 import static org.hamcrest.Matchers.equalTo;
 
-/**
- *
- */
 public class XContentBuilderTests extends ESTestCase {
     public void testPrettyWithLfAtEnd() throws Exception {
         ByteArrayOutputStream os = new ByteArrayOutputStream();
@@ -350,4 +347,33 @@ public class XContentBuilderTests extends ESTestCase {
                 "}", string.trim());
     }
 
+    public void testWriteMapWithNullKeys() throws IOException {
+        XContentBuilder builder = XContentFactory.contentBuilder(randomFrom(XContentType.values()));
+        try {
+            builder.map(Collections.singletonMap(null, "test"));
+            fail("write map should have failed");
+        } catch(IllegalArgumentException e) {
+            assertThat(e.getMessage(), equalTo("field name cannot be null"));
+        }
+    }
+
+    public void testWriteMapValueWithNullKeys() throws IOException {
+        XContentBuilder builder = XContentFactory.contentBuilder(randomFrom(XContentType.values()));
+        try {
+            builder.value(Collections.singletonMap(null, "test"));
+            fail("write map should have failed");
+        } catch(IllegalArgumentException e) {
+            assertThat(e.getMessage(), equalTo("field name cannot be null"));
+        }
+    }
+
+    public void testWriteFieldMapWithNullKeys() throws IOException {
+        XContentBuilder builder = XContentFactory.contentBuilder(randomFrom(XContentType.values()));
+        try {
+            builder.field("map", Collections.singletonMap(null, "test"));
+            fail("write map should have failed");
+        } catch(IllegalArgumentException e) {
+            assertThat(e.getMessage(), equalTo("field name cannot be null"));
+        }
+    }
 }