Can't convert from BSON type array to string exception in MongoDB aggregation

I get this exception from Mongodb aggregation operation:

I need to show in the aggregation and this is what it looks like in the base of mongodb

cannot convert from BSON type string to date (16006)).

  {
    _id: { $substr: ["$gateway.reverse.errors.message", 0, 500] },
    acum: {
        $sum: 1
    }
  }

They help me?

Thanks you