Apache ProxyPass websocket (wss) Issue

<VirtualHost *:443>
     ServerName mysite.com
     ServerAlias www.mysite.com

     SSLEngine on
     SSLProxyEngine On
     ProxyRequests Off

     SSLCertificateFile /etc/apache2/ssl/mysite.com.crt
     SSLCertificateKeyFile /etc/apache2/ssl/mysite.com.key
     SSLCertificateChainFile /etc/apache2/ssl/ca.cer

     DocumentRoot /var/www/errorPages

     ErrorDocument 503 /503.html
     ProxyPass /503.html !

     ProxyPass / http://localhost:3999/
     ProxyPassReverse / http://localhost:3999/

RewriteEngine on
RewriteCond %{HTTP:UPGRADE} ^WebSocket$ [NC]
RewriteCond %{HTTP:CONNECTION} ^Upgrade$ [NC]
RewriteRule .* ws://localhost:3999%{REQUEST_URI} [P]

</VirtualHost>