The document “Untitled” could not be saved as “Untitled”.

hi,in the chapter that explain the save\open method for the shopping list app i have a problem:
i write the implementation like this:

-(BOOL)writeToURL:(NSURL )absoluteURL oftype:(NSString *)typeName error:(NSError *)outError

{ return[shoppingListArray writeToURL:absoluteURL atomically:YES];
}

-(BOOL)readFromURL:(NSURL )absoluteURL ofType:(NSString *)typeName error:(NSError *)outError

{

shoppingListArray = [[NSMutableArray alloc] initWithContentsOfURL:absoluteURL];

return YES;

}

but when i try to save the file as:”untitled” the system display:

The document “Untitled” could not be saved as “Untitled”.

can anyone help me?
thanks