Gray box on UIWebView Screenshot

I save a webview screenshot to a file using this method:

UIGraphicsBeginImageContext(webview.bounds.size);     
[webview.layer renderInContext:UIGraphicsGetCurrentContext()];
UIImage *screenImage = UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();

[imageDat开发者_运维问答a writeToFile:imagePath atomically:YES]; 

But sometimes it's have a gray area in it. How i remove that a gray area?