MongoDb group sum by month unable to execute aggregate

I have an aggregation script which at the last stage results in the following output. I now want to group and sum by account category but it's telling me that I am somehow trying to convert from BSON type EOO to Date - which I caot see in my code anywhere.

{ 
"_id" : {
    "party_uuid" : "phildominickcompany", 
    "coection_uuid" : "5738fc661a21db15b5c45b49", 
    "account_balances_date" : ISODate("2016-06-30T10:00:00.000+0000"), 
    "object_origin_category" : "Bookkeeping", 
    "object_origin" : "Sage One"
}, 
"account_identifier" : "5010", 
"account_name" : "Cost of sales - materials", 
"account_category" : "Sales Expense", 
"account_type" : null, 
"account_value_type" : "debit", 
"account_value" : NumberInt(0)
}
{ 
"_id" : {
    "party_uuid" : "phildominickcompany", 
    "coection_uuid" : "5738fc661a21db15b5c45b49", 
    "account_balances_date" : ISODate("2016-07-31T10:00:00.000+0000"), 
    "object_origin_category" : "Bookkeeping", 
    "object_origin" : "Sage One"
}, 
"account_identifier" : "4000", 
"account_name" : "Sales Type A", 
"account_category" : "Sales Revenue", 
"account_type" : null, 
"account_value_type" : "credit", 
"account_value" : 57728.33
}
{ 
"_id" : {
    "party_uuid" : "phildominickcompany", 
    "coection_uuid" : "5738fc661a21db15b5c45b49", 
    "account_balances_date" : ISODate("2016-07-31T10:00:00.000+0000"), 
    "object_origin_category" : "Bookkeeping", 
    "object_origin" : "Sage One"
}, 
"account_identifier" : "5000", 
"account_name" : "Cost of sales - goods", 
"account_category" : "Sales Expense", 
"account_type" : null, 
"account_value_type" : "debit", 
"account_value" : NumberInt(10000)
}
{ 
"_id" : {
    "party_uuid" : "phildominickcompany", 
    "coection_uuid" : "5738fc661a21db15b5c45b49", 
    "account_balances_date" : ISODate("2016-07-31T10:00:00.000+0000"), 
    "object_origin_category" : "Bookkeeping", 
    "object_origin" : "Sage One"
}, 
"account_identifier" : "5010", 
"account_name" : "Cost of sales - materials", 
"account_category" : "Sales Expense", 
"account_type" : null, 
"account_value_type" : "debit", 
"account_value" : NumberInt(20000)
}
{ 
"_id" : {
    "party_uuid" : "phildominickcompany", 
    "coection_uuid" : "5738fc661a21db15b5c45b49", 
    "account_balances_date" : ISODate("2016-07-31T10:00:00.000+0000"), 
    "object_origin_category" : "Bookkeeping", 
    "object_origin" : "Sage One"
}, 
"account_identifier" : "6200", 
"account_name" : "Marketing", 
"account_category" : "Other Expense", 
"account_type" : null, 
"account_value_type" : "debit", 
"account_value" : NumberInt(1500)
}

The stage I'm trying to apply now is as follows. It is supposed to sum account_value by year and month. It's producing an error telling me I'm trying to go from an EOO to a date.

$group : {
_id: {"party_uuid" : "$party_uuid", 
  "account_category" : "$account_category",
  "account_balances_year" : {$year : "$account_balances_date"}, 
  "account_balances_month" : {$month : "$account_balances_date"}, 
  "account_category" : "$account_category",
  "account_type" : "$account_type",
  "object_origin_category" : "$object_origin_category", 
  "object_origin" : "$object_origin"},
  "month_value" : { $sum: "$account_value"}
}

Thanks, Matt