TypeError: Cannot read property '_id' of undefined (en el controlador del task que esta adjuntado abajo)

let taskSchema = new Schema({

     description: {

     type: String,
     required: [true, 'The description of the task is obligatory'],
     unique: true

     },

     completed: {

       type: Boolean,
       required: false,
       default: false

     },

    user: {

        type: Schema.Types.ObjectId,
        ref: 'User'

     }

 });`

let userSchema = new Schema({

 name: {

    type: String,
    required: [true, "The name is obligatory"]

 },

 email: {

    type: String,
    required: [true, "The email is obligatory"],
    unique: true

 },

 password: {

    type: String,
    required: [true, "The password is obligatory"]

 },

 identification: {

    type: String,
    required: [true, "The identification is obligatory"],
    unique: true

 },

 role: {

    type: String,
    enum: validRoles,
    required: false,
    default: "USER"

 },

 online: {

    type: Boolean,
    default: false

 },

 birthday: {

    type: Date,
    required: false,

 },

 tasks: {

    type: Array,
    required: false,
    default: []

 }

 });

let body = req.body;

      let task = new Task({

        (ACA ESTA EL ERROR!)user: req.user._id,
        description: body.description,
        completed: body.completed

    });

    task.save((err, task) => {

        if (err) {

            return res.status(500).json({

                ok: false,
                err

            });

        }

        res.json({

            ok: true,
            task,
            userUpdated

        });

    });