Aggregate command, 

[{"$match":{}},{"$densify":{"field":"age","partitionByFields":["age"],"range":{"bounds":"full","step":30}}}, {"$limit":10}, {"$limit":20}]

hits this tassert that should actually be a uassert or the $densify stage should be validated properly prior to this tassert. 

Consider adding a new uassert for the case that 'partitionByFields' contains the field to densify.