diff --git a/src/Type/Definition/IDType.php b/src/Type/Definition/IDType.php
index b45cfeb..89f4ff2 100644
--- a/src/Type/Definition/IDType.php
+++ b/src/Type/Definition/IDType.php
@@ -1,8 +1,10 @@
 <?php
 namespace GraphQL\Type\Definition;
 
+use GraphQL\Error\UserError;
 use GraphQL\Language\AST\IntValueNode;
 use GraphQL\Language\AST\StringValueNode;
+use GraphQL\Utils;
 
 /**
  * Class IDType
@@ -46,6 +48,12 @@ When expected as an input type, any string (such as `"4"`) or integer
         if ($value === false) {
             return 'false';
         }
+        if ($value === null) {
+            return 'null';
+        }
+        if (!is_scalar($value)) {
+            throw new UserError("String cannot represent non scalar value: " . Utils::printSafe($value));
+        }
         return (string) $value;
     }
 
diff --git a/src/Type/Definition/StringType.php b/src/Type/Definition/StringType.php
index d752ade..182643e 100644
--- a/src/Type/Definition/StringType.php
+++ b/src/Type/Definition/StringType.php
@@ -1,7 +1,9 @@
 <?php
 namespace GraphQL\Type\Definition;
 
+use GraphQL\Error\UserError;
 use GraphQL\Language\AST\StringValueNode;
+use GraphQL\Utils;
 
 /**
  * Class StringType
@@ -43,6 +45,12 @@ represent free-form human-readable text.';
         if ($value === false) {
             return 'false';
         }
+        if ($value === null) {
+            return 'null';
+        }
+        if (!is_scalar($value)) {
+            throw new UserError("String cannot represent non scalar value: " . Utils::printSafe($value));
+        }
         return (string) $value;
     }
 
diff --git a/tests/Type/ScalarSerializationTest.php b/tests/Type/ScalarSerializationTest.php
index b097a10..2f6ef18 100644
--- a/tests/Type/ScalarSerializationTest.php
+++ b/tests/Type/ScalarSerializationTest.php
@@ -119,6 +119,21 @@ class ScalarSerializationTest extends \PHPUnit_Framework_TestCase
         $this->assertSame('-1.1', $stringType->serialize(-1.1));
         $this->assertSame('true', $stringType->serialize(true));
         $this->assertSame('false', $stringType->serialize(false));
+        $this->assertSame('null', $stringType->serialize(null));
+
+        try {
+            $stringType->serialize([]);
+            $this->fail('Expected exception was not thrown');
+        } catch (UserError $e) {
+            $this->assertEquals('String cannot represent non scalar value: array', $e->getMessage());
+        }
+
+        try {
+            $stringType->serialize(new \stdClass());
+            $this->fail('Expected exception was not thrown');
+        } catch (UserError $e) {
+            $this->assertEquals('String cannot represent non scalar value: instance of stdClass', $e->getMessage());
+        }
     }
 
     /**