LINQ inner join returns "$project or $group does not support {document}."

Hi guys, I’m trying to implement inner join using LINQ, but I’m getting “$project or $group does not support {document}.”
What I need to do is join the customer for each location

customers

{
  {
  "name": "customer 1",
  "locations": [
    { "locationId": "1" },
    { "locationId": "2" },
    { "locationId": "3" }
  ]
},
  {
    "name": "customer 2",
    "locations": [
      { "locationId": "1" },
      { "locationId": "3" }
    ]
  }
} 

locations

{
  {
    "additionalInformation": "some info"
    "locationId": "1"
  },
  {
    "additionalInformation": "some info"
    "locationId": "2"
  }
  {
    "additionalInformation": "some info"
    "locationId": "3"
  }
}
(from c in _mongoContext.Customers.AsQueryable()
                                from location in c.Locations
                                join l in _mongoContext.Locations on location.LocationId equals l.LocationId
                                select new LocationSummery
                                {
                                    CustomerName = c.BusinessName,
                                    AdditionalInformation= location.AdditionalInformation,
                                })

Sorry the LINQ is

(from c in _mongoContext.Customers.AsQueryable()
from location in c.Locations
join l in _mongoContext.Locations on location.LocationId equals l.LocationId
select new LocationSummery
{
CustomerName = c.Name,
AdditionalInformation= location.AdditionalInformation,
})