title: Can not assign a float to NSNumber
body: <p>I have the following code in the m file of my root model:</p>
  
  <pre><code>-(id)init {
      if(self == [super init]) {
          self.rPrices = [[NSMutableArray alloc]init];
          self.rPrices = [NSArray arrayWithObjects:@"1", @"2", @"3", @"4", nil];
  }
      return self;
  }
  
  -(void)saveData:(NSMutableData *)data toFile:(NSString *)file {
  float nR4;
  nR4 = (some calculations ......)
  
  [self.rPrices addObject:[[NSNumber alloc] numberWithFloat:nR4]];
  
  }
  </code></pre>
  
  <p>I get the following error when I try to add the object:
  <strong>Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSPlaceholderNumber numberWithFloat:]: unrecognized selector sent to instance</strong> </p>
  
  <p>Thanks</p>