diff --git a/src/Executor/ExecutionResult.php b/src/Executor/ExecutionResult.php
index 074f200..7fc7523 100644
--- a/src/Executor/ExecutionResult.php
+++ b/src/Executor/ExecutionResult.php
@@ -14,15 +14,22 @@ class ExecutionResult
      * @var Error[]
      */
     public $errors;
+    
+    /**
+     * @var array[]
+     */
+    public $extensions;
 
     /**
      * @param array $data
      * @param array $errors
+     * @param array $extensions
      */
-    public function __construct(array $data = null, array $errors = [])
+    public function __construct(array $data = null, array $errors = [], array $extensions = [])
     {
         $this->data = $data;
         $this->errors = $errors;
+        $this->extensions = $extensions;
     }
 
     /**
@@ -35,6 +42,10 @@ class ExecutionResult
         if (!empty($this->errors)) {
             $result['errors'] = array_map(['GraphQL\Error', 'formatError'], $this->errors);
         }
+        
+        if (!empty($this->extensions)) {
+            $result['extensions'] = (array) $this->extensions;
+        }
 
         return $result;
     }
diff --git a/src/Type/Definition/InterfaceType.php b/src/Type/Definition/InterfaceType.php
index 7bf37c5..dd5a6e2 100644
--- a/src/Type/Definition/InterfaceType.php
+++ b/src/Type/Definition/InterfaceType.php
@@ -49,8 +49,9 @@ class InterfaceType extends Type implements AbstractType, OutputType, CompositeT
     public static function addImplementationToInterfaces(ObjectType $impl)
     {
         self::$_lazyLoadImplementations[] = function() use ($impl) {
+            /** @var self $interface */
             foreach ($impl->getInterfaces() as $interface) {
-                $interface->_implementations[] = $impl;
+                $interface->addImplementation($impl);
             }
         };
     }
@@ -66,6 +67,16 @@ class InterfaceType extends Type implements AbstractType, OutputType, CompositeT
         self::$_lazyLoadImplementations = [];
     }
 
+    /**
+     * Add a implemented object type to interface
+     *
+     * @param ObjectType $impl
+     */
+    protected function addImplementation(ObjectType $impl)
+    {
+        $this->_implementations[] = $impl;
+    }
+
     /**
      * InterfaceType constructor.
      * @param array $config
diff --git a/tests/Executor/ExecutionResultTest.php b/tests/Executor/ExecutionResultTest.php
new file mode 100644
index 0000000..1c942f7
--- /dev/null
+++ b/tests/Executor/ExecutionResultTest.php
@@ -0,0 +1,25 @@
+<?php
+namespace GraphQL\Tests\Executor;
+
+use GraphQL\Executor\ExecutionResult;
+
+class ExecutionResultTest extends \PHPUnit_Framework_TestCase
+{
+    public function testToArrayWithoutExtensions()
+    {
+        $executionResult = new ExecutionResult();
+
+        $this->assertEquals(['data' => null], $executionResult->toArray());
+    }
+
+    public function testToArrayExtensions()
+    {
+        $executionResult = new ExecutionResult(null, [], ['foo' => 'bar']);
+
+        $this->assertEquals(['data' => null, 'extensions' => ['foo' => 'bar']], $executionResult->toArray());
+
+        $executionResult->extensions = ['bar' => 'foo'];
+
+        $this->assertEquals(['data' => null, 'extensions' => ['bar' => 'foo']], $executionResult->toArray());
+    }
+}