title: No visible @interface for 'NSMutableDictionary' declares the selector 'addObjects:forKey'
body: <p>My code gives an error saying</p>
  
  <blockquote>
    <p>No visible interface for 'NSMutableDictionary' declares the selector 'addObjects:forKey:'</p>
  </blockquote>
  
  <p>.h</p>
  
  <pre><code>@interface ViewController : UIViewController{
      NSMutableDictionary *credtialsDictionary;
  }
  -(IBAction)registerlogin;
  @end
  </code></pre>
  
  <hr>
  
  <p>.m</p>
  
  <pre><code>@implementation ViewController
  
  -(IBAction)registerlogin{
  
      [credtialsDictionary addObjects:passwordFieldregister.text forKey:usernameFieldregister];
  }
  @end
  </code></pre>
  
  <p>I can't figure out why this <code>IBAction</code> isn't letting me add objects to the <code>NSMutableDictionary</code></p>