children()->count() error

[Oreolek]

ERROR: column "categories.lft" must appear in the GROUP BY clause or be used in an aggregate function

PHP code: $model->children()->count()

SQL code: SELECT COUNT(*) FROM "modeltable" WHERE (("lft" > 2) AND ("rgt" < 3)) AND ("root"=375) ORDER BY "lft"

Workaround (if this won't get a fix): children()->orderBy(NULL)->count()