Extract jSignature image from UIWebView

I am trying to extract signature bitmap from UIWebView.

This is HTML code:

<canvas class="jSignature" width="200" height="120" style="margin: 0px; padding: 0px; border: none; height: 120px; width: 200px;"></canvas>

This code works in JS:

$('.signature').jSignature('getData','image')

This is my piece of code:

NSString *sign = [self.webView stringByEvaluatingJavaScriptFromString:@"$('.signature').jSignature('getData','image')"];

So sign is:

<object returned empty description>

Is there any way to get error message from stringByEvaluatingJavaScriptFromString to debug this problem?