diff --git a/src/main/java/graphql/schema/GraphQLInputObjectType.java b/src/main/java/graphql/schema/GraphQLInputObjectType.java
index 9929f93493..e10eddadb7 100644
--- a/src/main/java/graphql/schema/GraphQLInputObjectType.java
+++ b/src/main/java/graphql/schema/GraphQLInputObjectType.java
@@ -1,5 +1,6 @@
 package graphql.schema;
 
+import com.google.common.collect.ImmutableCollection;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import graphql.Directives;
@@ -152,7 +153,8 @@ public GraphQLInputObjectField getFieldDefinition(String name) {
 
     @Override
     public List<GraphQLInputObjectField> getFieldDefinitions() {
-        return ImmutableList.copyOf(fieldMap.values());
+        ImmutableCollection<GraphQLInputObjectField> values = fieldMap.values();
+        return values instanceof ImmutableList<?> ? (ImmutableList<GraphQLInputObjectField>) values : ImmutableList.copyOf(values);
     }
 
     public InputObjectTypeDefinition getDefinition() {