diff --git a/src/github.com/matrix-org/dendrite/roomserver/storage/membership_table.go b/src/github.com/matrix-org/dendrite/roomserver/storage/membership_table.go
index 8632d391c..725e5b8d9 100644
--- a/src/github.com/matrix-org/dendrite/roomserver/storage/membership_table.go
+++ b/src/github.com/matrix-org/dendrite/roomserver/storage/membership_table.go
@@ -50,6 +50,8 @@ CREATE TABLE IF NOT EXISTS roomserver_membership (
 );
 `
 
+// Insert a row in to membership table so that it can be locked by the
+// SELECT FOR UPDATE
 const insertMembershipSQL = "" +
 	"INSERT INTO roomserver_membership (room_nid, target_nid)" +
 	" VALUES ($1, $2)" +