diff --git a/src/Execution/Request.php b/src/Execution/Request.php
index 5b3da1a0..88b7be72 100644
--- a/src/Execution/Request.php
+++ b/src/Execution/Request.php
@@ -66,6 +66,13 @@ public function __construct($data = [], $variables = [])
                 if (!array_key_exists($ref->getName(), $variables)) {
                     /** @var Variable $variable */
                     $variable = $ref->getVariable();
+                    /**
+                     * If $variable is null, then it was not declared in the operation arguments
+                     * @see https://graphql.org/learn/queries/#variables
+                     */
+                    if (is_null($variable)) {
+                        throw new InvalidRequestException(sprintf("Variable %s hasn't been declared", $ref->getName()), $ref->getLocation());
+                    }
                     if ($variable->hasDefaultValue()) {
                         $variables[$variable->getName()] = $variable->getDefaultValue()->getValue();
                         continue;
diff --git a/tests/Issues/Issue238/Issue238Test.php b/tests/Issues/Issue238/Issue238Test.php
new file mode 100644
index 00000000..934d1e37
--- /dev/null
+++ b/tests/Issues/Issue238/Issue238Test.php
@@ -0,0 +1,57 @@
+<?php
+
+namespace Youshido\Tests\Issue238;
+
+use PHPUnit\Framework\TestCase;
+use Youshido\GraphQL\Schema\Schema;
+use Youshido\GraphQL\Execution\Processor;
+use Youshido\GraphQL\Type\Scalar\IntType;
+use Youshido\GraphQL\Execution\ResolveInfo;
+use Youshido\GraphQL\Type\Object\ObjectType;
+use Youshido\GraphQL\Type\Scalar\StringType;
+
+class Issue238Test extends TestCase
+{
+    public function testNotDefinedVariableThrowsException()
+    {
+        $schema = new Schema([
+            'query' => new ObjectType([
+                'name'   => 'RootQuery',
+                'fields' => [
+                    'currentUser' => [
+                        'type'    => new StringType(),
+                        'args'    => [
+                            'age' => [
+                                'type'         => new IntType(),
+                                'defaultValue' => 20,
+                            ],
+                        ],
+                        'resolve' => static function ($source, $args, ResolveInfo $info) {
+                            return 'Alex age ' . $args['age'];
+                        },
+                    ],
+                ],
+            ]),
+        ]);
+
+        $processor = new Processor($schema);
+
+        /**
+         * If using a variable in a query, it must be defined in the operation
+         * $properQuery = 'query GetUserAge($age:Int) { currentUser(age:$age) }';
+         * When not declaring the variable in the operation, it must return a nice error message
+         * (instead of throwing a RuntimeException)
+         */
+        $unproperQuery = '{ currentUser(age:$age) }';
+        $response = $processor->processPayload($unproperQuery)->getResponseData();
+        if ($this->assertArrayHasKey(
+            'errors',
+            $response
+        )) {
+            $this->assertArraySubset(
+                ['message' => 'Variable age hasn\'t been declared'],
+                $response['errors']
+            );
+        }
+    }
+}