Recoverable fatal error

In parse method of CSVParser there is

 if (!$header) {
        return;
      }

So it can return null.

In import method of FeedsSource.inc there is

$parser_result = $this->importer->parser->parse($this, $this->fetcher_result);
module_invoke_all('feeds_after_parse', $this, $parser_result);

if $parser_result is null and you have your custom after_parse hook(what actually happened to me) null will be passed as the second argument and you will get fatal error like:
Recoverable fatal error: Argument 2 passed to custom_module_feeds_after_parse() must be an instance of FeedsParserResult