title: Assertion failure in UICollectionView
body: <p>I'm getting this error: </p>
  
  <pre><code>*** Assertion failure in -[UICollectionView _dequeueReusableViewOfKind:withIdentifier:forIndexPath:viewCategory:], /SourceCache/UIKit_Sim/UIKit-2935.137/UICollectionView.m:3241
  </code></pre>
  
  <p>Two methods:</p>
  
  <pre><code>- (id)initWithCollectionViewLayout:(UICollectionViewFlowLayout *)layout {
      if (self = [super initWithCollectionViewLayout:layout]) {
      [self.collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"CELL"];
      [self.collectionView setBackgroundColor:[UIColor clearColor]];
      }
      return self;
  }
  
  - (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
      UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"CELL" forIndexPath:indexPath];
      cell.backgroundColor = [UIColor whiteColor];
  
      UIImageView *backgroundView= [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"bg"]];
      cell.backgroundView = backgroundView;
  
      return cell;
  }
  
  - (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView {
      return 1;
  }
  </code></pre>
  
  <p>I've also checked for typos or spaces in my identifiers both in code and my Storyboard.</p>