diff --git a/core/src/main/java/org/elasticsearch/index/mapper/MapperService.java b/core/src/main/java/org/elasticsearch/index/mapper/MapperService.java
index 4088f654d80..4b162b0423d 100755
--- a/core/src/main/java/org/elasticsearch/index/mapper/MapperService.java
+++ b/core/src/main/java/org/elasticsearch/index/mapper/MapperService.java
@@ -64,7 +64,6 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
-import java.util.HashSet;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
@@ -76,6 +75,7 @@ import java.util.function.Function;
 import static java.util.Collections.emptySet;
 import static java.util.Collections.unmodifiableSet;
 import static org.elasticsearch.common.collect.MapBuilder.newMapBuilder;
+import static org.elasticsearch.common.util.set.Sets.newHashSetCopyWith;
 
 /**
  *
@@ -288,10 +288,7 @@ public class MapperService extends AbstractIndexComponent implements Closeable {
                 }
                 mappers = newMapBuilder(mappers).put(mapper.type(), mapper).map();
                 if (mapper.parentFieldMapper().active()) {
-                    Set<String> parentTypesCopy = new HashSet<String>();
-                    parentTypesCopy.addAll(parentTypes);
-                    parentTypesCopy.add(mapper.parentFieldMapper().type());
-                    parentTypes = unmodifiableSet(parentTypesCopy);
+                    parentTypes = unmodifiableSet(newHashSetCopyWith(parentTypes, mapper.parentFieldMapper().type()));
                 }
                 assert assertSerialization(mapper);
                 return mapper;