Infinity 404 message shows not secure.

I uploaded a 404.shtml page but I assume this is being blocked by free cloudflare htpps account associated with the site. The 404.shtml exceeds the 3 page requirement of free cloudflare (the site has 3 pages plus the 404).
When a 404 is generated it loads the InfinityFree 404 message ( which then generates a Avast security block message saying:
“Warning” This site has been marked as a phishing site." 
Other than the recommendation to stay in the 3 page limit of Cloudflare is there any solution for this - any ideas would be welcome. Thank you.

Please note that our servers don't look at files of a particular name to figure out what to use as the error page. The error pages are controlled using ErrorDocument lines in your .htaccess file. You can use those rules to use any page on your website as your error page. Alternatively, the Error Pages tool in the control panel can set that up for you.

Thanks for reporting the Avast error though, it seems they have incorrectly marked our site as harmful (again). I've submitted a review request with them, but it usually helps if their actual customers submit reports as well.