Modifying a crossdomain XML policy to accept null origin headers

I am currently dealing with an error that reads

Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'null' is therefore not allowed access. The response had HTTP status code 404.

The request is working fine on my server, but breaks on my client. I recently found this file, public/crossdomain.xml

<?xml version="1.0"?>
<!DOCTYPE cross-domain-policy SYSTEM "http://ift.tt/1d1B8yI">
<cross-domain-policy>
  <!-- Read this: http://ift.tt/18sUytt -->

  <!-- Most restrictive policy: -->
  <!-- <site-control permitted-cross-domain-policies="none"/> -->

  <!-- Least restrictive policy: -->

  <site-control permitted-cross-domain-policies="all"/>
  <allow-access-from domain="*" to-ports="*" secure="false"/>
  <allow-http-request-headers-from domain="*" headers="*" secure="false"/>

</cross-domain-policy>

While this looks like the fix I have been seeking, I am still being thrown the same error. There are no issues like this server side, and my client now largely matches the configuration of my server. Why would this policy fail to resolve the error? Do I need to configure this policy further to accept the required headers?