Uppercase Header in WAF Rule

I want to set an Uppercase header in WAF rule like:

(all(http.request.headers["X-Custom-Header"][*] ne "test"))

But i will get the following error:

is not a valid value for expression because it is not possible to use an uppercase string Content-type as key for the field http.request.headers (Code: 20127) 

The problem is that the client only sends headers in uppercase, and it’s using HTTP/1.0. Even though I make it header lowercase in the rule, the request will be blocked if the incoming header is not in lowercase.