Setting Content-Length header with PHP filesize() gives ERR_SPDY_PROTOCOL_ERROR in Chrome

Question

I'm on a shared hosting site, using Apache 2.2.22 and PHP 7.0.27, gzipped output.

Reading a file with PHP and setting the headers with

header("Content-Length: ".filesize($filename));

gives ERR_SPDY_PROTOCOL_ERROR when viewed in Chrome, with the SSL on, works without SSL.

I know that this means that the given length in the header doesn't match with the real length, but why? How can I get the correct length for the header?