diff --git a/src/modules/usrloc/README b/src/modules/usrloc/README
index b8faee9efb8..74a96729515 100644
--- a/src/modules/usrloc/README
+++ b/src/modules/usrloc/README
@@ -924,7 +924,13 @@ modparam("usrloc", "db_raw_fetch_type", 1)
 3.42. db_insert_null (int)
 
    If set to 1, the insert operation to database will add null values in
-   the sql statement.
+   the statement. It has to be set to 1 for the database systems that do
+   not have table definitions that create automatically the null fields
+   (e.g., db_mongodb) for each stored record.
+
+   When set to 0, the fiels that default to null are not added to the DB
+   insert statement if they don't have a different value, making the query
+   smaller.
 
    Default value is “0” (don't add null fields in insert statement).