CFURLCreateDataAndPropertiesFromResource failed with error code -15

I'm trying to lead a TIFF image into a CGImageSource using CGImageSourceCreateWithURL. My URL is correct, is %20 encoded, is a path to an existing file, that file is a TIFF. However, the line:

 NSString *fullPath = [[[fileNames objectAtIndex:pageNum - 1] string]
 stringByAddingPercentEscapesUsingEncoding: 
 NSUTF8StringEncoding];
 NSString* urlStr = [NSString stringWithFormat: @"file:/%@", fullPath];
 NSURL * url = [NSURL URLWithString: fullPath];
 CGImageSourceRef src = CGImageSourceCreateWithURL (url, NULL);

gives me the following error:
Wed Aug 4 20:17:20 Brians-mini.local DocKeep[17199] <Error>: CGImageSourceCreateWithURL: CFURLCreateDataAndPropertiesFromResource failed with error code -15.

anyone know what error -15 is? or where I can find it?

thanks