Ios – Terminating app due to uncaught exception ‘NSInvalidArgumentException’, reason: ‘-[__NSCFString size]: unrecognized selector sent to instance

I am creating UIButton dynamically in the FOR loop as follows :

     CGRect workingFrame = imgscrollView.frame;
      workingFrame.origin.x = 0;
      workingFrame.origin.y = 0;

   for (int i=0 ; i < self.currentDetails.arrayOfImages.count ; i++)
   {
    UIButton *imageBtn = [[UIButton alloc] init];
    [imageBtn setImage:image forState:UIControlStateNormal];
    [imageBtn setUserInteractionEnabled:TRUE];
    imageBtn.layer.cornerRadius = 8;
    imageBtn.layer.borderWidth = 1;
    imageBtn.layer.borderColor = [UIColor whiteColor].CGColor;
    imageBtn.layer.masksToBounds = YES;
    imageBtn.clipsToBounds = YES;
    [imageBtn setContentMode:UIViewContentModeScaleAspectFill];
    [imageBtn addTarget:self action:@selector(changeButtonImage:) forControlEvents:UIControlEventTouchUpInside];
    [imageBtn setTag:i];
    [imgscrollView addSubview:imageBtn];

     imageBtn.frame = CGRectMake(workingFrame.origin.x+20, workingFrame.origin.y, 145, 140);
   }

But at the time of setting its frame

imageBtn.frame = CGRectMake(workingFrame.origin.x+20, workingFrame.origin.y, 145, 140);

i am getting following error and it crashes :

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFString size]: unrecognized selector sent to instance

I have searched for this but could not get the solution.

Please help me.

Thanks…