Either one of your values is guaranteed to be nil, or one of your values is not an object pointer, because the following code yields no error at all:
- (NSString *)description
{
return [@{@"filePath": @"",
@"fileName": @"",
@"fileAttributes": @"",
@"creationDate": @"",
@"modificationDate": @"",
@"fileSize": @"",
@"age": @"",
@"isArchived": @""} description];
}