Params Not Built Correctly

I'm building an application for requesting equipment. I have a form which includes three models. Following the Railscast on Multi-Model forms, I got this working for the most part, except I don't get data back for the 3rd level model in my params.

I learned about raw_post and parsed through that and the data is there, but it is not coming through to params. Here is the debug output.

_method=put
request[line_item_attributes][][equipment_type_id]=1
request[line_item_attributes][][count]=3
request[line_item_attributes][][id]=17
request[line_item_attributes][][equipment_line_item_attributes][][equipment_item_id]=4
request[line_item_attributes][][equipment_line_item_attributes][][equipment_item_id]=10
request[line_item_attributes][][equipment_type_id]=4
request[line_item_attributes][][count]=1
request[line_item_attributes][][id]=18
request[line_item_attributes][][equipment_line_item_attributes][][equipment_item_id]=9
request[line_item_attributes][][equipment_type_id]=6
request[line_item_attributes][][count]=1
request[line_item_attributes][][id]=19
commit=Update

{
"commit"=>"Update",
"_method"=>"put",
"action"=>"update",
"request"=>
   {"line_item_attributes"=>
     [{"equipment_type_id"=>"1",
       "id"=>"17",
       "count"=>"3",
       "equipment_line_item_attributes"=>[]
       },
       {"equipment_type_id"=>"4",
       "id"=>"18",
       "count"=>"1",
       "equipment_line_item_attributes"=>[]
       },
       {"equipment_type_id"=>"6",
       "id"=>"19",
       "count"=>"1",
       "equipment_line_item_attributes"=>[]
     }]
   }
}

Are there any outstanding issues with this sort of thing? My HTML is obviously being built correctly to get the correct data back.

Does anyone have any ideas on what might be the problem?

Alan