CFURLCreateDataAndPropertiesFromResource failed with error code -15

Question

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

ETA: I should also mention that I don't have this problem if the path doesn't have spaces in it... (Spaces in filenames are an abomination unto the lord, but I suppose we have to live with them... B-)