Functional test for aggregate command failure

tests/Operation/AggregateFunctionalTest.php

+<?php
+
+namespace MongoDB\Tests\Operation;
+
+use MongoDB\Operation\Aggregate;
+
+class AggregateFunctionalTest extends FunctionalTestCase
+{
+    /**
+     * @expectedException MongoDB\Driver\Exception\RuntimeException
+     */
+    public function testUnrecognizedPipelineState()
+    {
+        $server = $this->getPrimaryServer();
+        $operation = new Aggregate($this->getDatabaseName(), $this->getCollectionName(), [['$foo' => 1]]);
+        $operation->execute($server);
+    }
+}