diff --git a/src/GraphQL.php b/src/GraphQL.php
index 8574236..d0d3ec6 100644
--- a/src/GraphQL.php
+++ b/src/GraphQL.php
@@ -18,7 +18,7 @@ class GraphQL
 {
     /**
      * @param Schema $schema
-     * @param $requestString
+     * @param string|DocumentNode $requestString
      * @param mixed $rootValue
      * @param array|null $variableValues
      * @param string|null $operationName
@@ -41,7 +41,7 @@ class GraphQL
 
     /**
      * @param Schema $schema
-     * @param $requestString
+     * @param string|DocumentNode $requestString
      * @param null $rootValue
      * @param null $variableValues
      * @param null $operationName
diff --git a/src/Language/Source.php b/src/Language/Source.php
index 5083f40..d831b29 100644
--- a/src/Language/Source.php
+++ b/src/Language/Source.php
@@ -1,6 +1,8 @@
 <?php
 namespace GraphQL\Language;
 
+use GraphQL\Utils;
+
 class Source
 {
     /**
@@ -20,6 +22,11 @@ class Source
 
     public function __construct($body, $name = null)
     {
+        Utils::invariant(
+            is_string($body),
+            'GraphQL query body is expected to be string, but got ' . Utils::getVariableType($body)
+        );
+
         $this->body = $body;
         $this->length = mb_strlen($body, 'UTF-8');
         $this->name = $name ?: 'GraphQL';