Mongoose populate returning null

My code contains 2 models, Content and Movie. Movie has a reference to Content. In 'Movie' query when i try to populate Content', null is returned.

1. Content Model

   'use strict';
   
   var mongoose = require('mongoose'),
       validators = require('./validators'),
       Schema = mongoose.Schema;
   
   var contentType = ['M'];
   
   var ContentSchema = new Schema({
       createdOn: {
           type: Date,
           default: Date.now
       },
       name: {
           type: String,
           required: true,
           unique: true,
           trim: true
       },
       type: {
           type: String,
           required: true,
           enum: contentType,
           trim: true
       },
       releaseDate: {
           type: Date
       },
       genres: [{
           type: Schema.Types.ObjectId,
           ref: 'Genre'
       }],
       language: {
           type: Schema.Types.ObjectId,
           ref: 'Language'
       },
       imagePath: {
           type: String
       },
       filePath: {
           type: String
       }
   });
   
   mongoose.model('Content', ContentSchema);
   

2. Movie Model

   var mongoose = require('mongoose'),
       validators = require('./validators'),
       Schema = mongoose.Schema;
   
   var MovieSchema = new Schema({
       createdOn: {
           type: Date,
           default: Date.now
       },
       contentId: {
           type: Schema.Types.ObjectId,
           ref: 'Content'
       },
       cast: {
           type: [String]
       },
       synopsis: {
           type: String
       },
       length: {
           type: Number
       },
       director: {
           type: String
       }
   });
   
   mongoose.model('Movie', MovieSchema);
   

This is my controller on server side:

Movie.find().populate('contentId').exec(function (err, movies) {
    if (err) {
        return res.json(500, {
            error: 'cannot list movies'
        });
    }
    res.json(movies);
});

Output:

[
    {
        "_id": "540dcdda98fcaefbaf26fd72",
        "contentId": null,
        "synopsis": "Nice Movie",
        "length": 140,
        "director": "Foo Bar",
        "cast": [],
        "createdOn": "2014-09-08T16:38:19.275Z"
    }
]

Why is the output null even though I can see contentId in output if i remove populate from my query.