title: AssertionError: Target cannot be null or undefined
body: <p>I am working on this testing suite that is frustrating me because I continue to get this error:</p>
  
  <pre><code>1) BlogPost API resource
         GET endpoint
           should return all existing posts:
       AssertionError: Target cannot be null or undefined.
        at D:\Projects\Thinkful\mongooseBlog02\blog-app-mongoose-challenge-solution\test\test-blog-integration.js:128:54
        at &lt;anonymous&gt;
        at process._tickCallback (internal/process/next_tick.js:188:7)
  </code></pre>
  
  <p>This is in reference to line 128 in my <code>test-blog-integration.js</code> file:</p>
  
  <pre><code>describe('GET endpoint', function() {
  
      it('should return all existing posts', function() {
        // strategy:
        //    1. get back all restaurants returned by by GET request to `/restaurants`
        //    2. prove res has right status, data type
        //    3. prove the number of restaurants we got back is equal to number
        //       in db.
        //
        // need to have access to mutate and access `res` across
        // `.then()` calls below, so declare it here so can modify in place
        let res;
        return chai.request(app)
          .get('/posts')
          .then(function(_res) {
            // so subsequent .then blocks can access response object
            res = _res;
            expect(res).to.have.status(200);
            console.log("testKC");
            // otherwise our db seeding didn't work
            expect(res.body.posts).to.have.lengthOf.at.least(1);
            return allPosts.count();
          })
          .then(function(count) {
            expect(res.body.posts).to.have.lengthOf(count);
          });
      });
  </code></pre>
  
  <p>I have tried <code>posts</code>, <code>allPosts</code>, but I continue to get the same error. I got <code>allPosts</code> from my <code>models.js</code> file:</p>
  
  <pre><code>'use strict';
  
  const mongoose = require('mongoose');
  mongoose.Promise = global.Promise;
  
  const blogPostSchema = mongoose.Schema({
    author: {
      firstName: String,
      lastName: String
    },
    title: {type: String, required: true},
    content: {type: String},
    created: {type: Date, default: Date.now()}
  });
  
  
  blogPostSchema.virtual('authorName').get(function() {
    return `${this.author.firstName} ${this.author.lastName}`.trim();
  });
  
  blogPostSchema.methods.serialize = function() {
    return {
      id: this._id,
      author: this.author,
      content: this.content,
      title: this.title,
      created: this.created
    };
  };
  
  const BlogPost = mongoose.model('allPosts', blogPostSchema);
  
  module.exports = { BlogPost };
  </code></pre>