Elastic Beanstalk Configuration Error when setting proxy to Apache

As per these instructions I attempted to reconfigured Elastic Beanstalk to use the Apache HTTPD Proxy, as I am having no luck getting nginx to proxy websocket requests to my Spring Boot Java Application from Firefox (it works with Chrome):

option_settings:
  aws:elasticbeanstalk:environment:proxy:
    ProxyServer: apache

When I deploy this version I get the following errors:

Failed to deploy application.
Invalid option specification (Namespace: 'aws:elasticbeanstalk:environment:proxy', OptionName: 'ProxyServer'): Unknown configuration setting.
"option_settings" in one of the configuration files failed validation. More details to follow.

The option is configured in the deployed zip file in the file .ebextensions/httpd-proxy.config as per the instructions.

Many thanks, Mark.