Apache httpd will not return a custom error page on status code 400

Issue

I specified custom error page (html or CGI) to ErrorDocument on status code 400 like the followings:

ErrorDocument 400 /somepage.html

or

ErrorDocument 400 /cgi-bin/error.cgi

But Apache httpd did not return a custom error page for "400 Bad Reqeust" error response when a request is malformed. for example:

# sending an HTTP/1.1 request without Host header
$ curl -v http://localhost/ -H "Host:" 

# sending a large request header 
$ curl -v http://localhost/ -H "key: `perl -e 'print "a"x10000'`"