title: iOS & Stackmob - [NSNull length]: unrecognized selector sent to instance
body: <p>In my iOS app I'm trying to update user information in the database (with Stackmob), but I keep getting "unrecognized selector sent to instance." </p>
  
  <pre><code>- (IBAction)save:(UIButton *)sender {
  
  NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"User"];
  NSPredicate *predicte = [NSPredicate predicateWithFormat:@"username == %@", self.username];
  [fetchRequest setPredicate:predicte];
  
  [self.managedObjectContext executeFetchRequest:fetchRequest onSuccess:^(NSArray *results) {
  
      NSManagedObject *todoObject = [results objectAtIndex:0];
      [todoObject setValue:@"example@gmail.com" forKey:@"email"];
  
      [self.managedObjectContext saveOnSuccess:^{
          NSLog(@"You updated the todo object!");
      } onFailure:^(NSError *error) {
          NSLog(@"There was an error! %@", error);
      }];
  
  } onFailure:^(NSError *error) {
  
      NSLog(@"Error fetching: %@", error);
  
  }];
  }
  </code></pre>
  
  <p>Here's the full error I'm getting:</p>
  
  <pre><code>-[NSNull length]: unrecognized selector sent to instance 0x1ec5678
  
  2013-07-21 12:01:25.773 [29207:c07] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', 
  reason: '-[NSNull length]: unrecognized selector sent to instance 0x1ec5678'
  </code></pre>
  
  <p>Thanks in advance.</p>