diff --git a/source/applications/aggregation.txt b/source/applications/aggregation.txt
index cc8320d47ab..04cff713ae1 100644
--- a/source/applications/aggregation.txt
+++ b/source/applications/aggregation.txt
@@ -143,7 +143,7 @@ command:
      } },
      { $unwind : “$tags” },
      { $group : {
-        _id : { tags : 1 },
+        _id : { tags : "$tags" },
         authors : { $addToSet : “$author” }
      } }
    );