'NSInvalidArgumentException', reason: '*** -[NSPlaceholderString initWithFormat:locale:arguments:]: nil argument'

Question

va_list arglist;
if (!formatstring) return;

va_start(arglist, formatstring);
id outstring = [[[NSString alloc] initWithFormat:formatstring arguments:arglist] autorelease];
va_end(arglist);

UIAlertView *av = [[[UIAlertView alloc] initWithTitle:outstring message:nil delegate:nil cancelButtonTitle:@"OK"otherButtonTitles:nil] autorelease];
av.tag=2;
[av show];

i am getting this error in console.

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '* -[NSPlaceholderString initWithFormat:locale:arguments:]: nil argument'