diff --git a/lib/Doctrine/MongoDB/Aggregation/Expr.php b/lib/Doctrine/MongoDB/Aggregation/Expr.php
index 7c50a1d3..4f2454e9 100644
--- a/lib/Doctrine/MongoDB/Aggregation/Expr.php
+++ b/lib/Doctrine/MongoDB/Aggregation/Expr.php
@@ -242,7 +242,7 @@ private function ensureArray($expression)
      */
     public function dateToString($format, $expression)
     {
-        return $this->operator('$dateToString', array($format, $expression));
+        return $this->operator('$dateToString', array('format' => $format, 'date' => $expression));
     }
 
     /**
diff --git a/tests/Doctrine/MongoDB/Tests/Aggregation/ExprTest.php b/tests/Doctrine/MongoDB/Tests/Aggregation/ExprTest.php
index ad006cba..177ce563 100644
--- a/tests/Doctrine/MongoDB/Tests/Aggregation/ExprTest.php
+++ b/tests/Doctrine/MongoDB/Tests/Aggregation/ExprTest.php
@@ -81,7 +81,7 @@ public function testDateToString()
         $expr = new Expr();
 
         $this->assertSame($expr, $expr->dateToString('%Y-%m-%d', '$dateField'));
-        $this->assertSame(array('$dateToString' => array('%Y-%m-%d', '$dateField')), $expr->getExpression());
+        $this->assertSame(array('$dateToString' => array('format' => '%Y-%m-%d', 'date' => '$dateField')), $expr->getExpression());
     }
 
     public function testDayOfMonth()