$pull is not working

I want to delete am item reference from restaurant when that item is deleted. What i did is, i made a method to pull out that item from restaurant but its not working. I found error like MongoError: Cannot apply $pull/$pullAll modifier to non-array.

here is my restaurant schema :

var Restaurant = new Schema({
    name: [languageData],
    description: [languageData],
    phone: String,
    email: String, 
    items: [{ type: ObjectId, ref: 'Item' }]
});

exports.restaurantModel = mongoose.model("Restaurant", Restaurant); 

and this this my function to pull item from restaurant :

restaurantModel.update({'_id' : restId}, {'$pull': {'items': itemId}}, function(err,data){
    if (!err) {
        console.log("Refrence of Item is updated"+data);
    } else {
        console.log("Error : " +err);
    }
});

i cant understand what is the problem. i have even tried to use multi : true and upsert : false but still there is an error.. Please help me out of this.