无法在Golang mongo驱动程序中将字符串编组为BSON文档错误

I am trying to read some data from MongoDB in go using gopkg.in/mgo.v2 driver. i have written the following code to extract the information form MongoDB.

my go struct

type NetworkUser struct {
    Id         bson.ObjectId   `bson:"_id,omitempty"        json:"-"`
    FirstName  string          `bson:"firstName"            json:"firstName"`
    MiddleName string          `bson:"middleName,omitempty" json:"middleName,omitempty"`
    LastName   string          `bson:"lastName"             json:"lastName"`
    Inserted   time.Time       `bson:"inserted"             json:"-"`
}

then, i have written the following code for reading from the mongo

//Connect to the Collection and Execute query
_collection := _session.DB(database).C(collection)

//Executing the query 
var results []NetworkUser
err = _collection.Find(query).All(&results)

//Handle the errors
if err != nil {
    fmt.Println(err)
    panic(err)
}

When i run the code, i am getting the following error : Can't marshal string as a BSON document

Please let me know, if anyone come across this error and how to fix it..