diff --git a/tests/views_groupby.test b/tests/views_groupby.test
index dd0de76..4e09256 100644
--- a/tests/views_groupby.test
+++ b/tests/views_groupby.test
@@ -109,11 +109,12 @@ class ViewsQueryGroupByTest extends ViewsSqlTest {
 
   /**
    * @param string|null $group_by
-   *   Which group_by function should be used, for example sum or count.
+   *   (optional) Which group_by function should be used, for example sum or
+   *   count. If omitted, the aggregation is tested with no group function.
    * @param array|null $values
-   *   Expected values.
+   *   (optional) Expected values.
    */
-  function GroupByTestHelper($group_by, $values = NULL) {
+  function GroupByTestHelper($group_by = NULL, $values = NULL) {
     // Create 4 nodes of type1 and 3 nodes of type2
     $type1 = $this->drupalCreateContentType();
     $type2 = $this->drupalCreateContentType();