Unreported 400 Bad Request Errors

The following two curl commands trigger 400 Bad Request errors which don’t appear in the ErrorLog and aren’t handled by the custom ErrorDocument

curl localhost -H ";"

> 400 Bad Request
> Your browser sent a request that this server could not understand.

This one is slightly different:

curl localhost -H "Host;"

> 400 Bad Request
> Your browser sent a request that this server could not understand.
> Additionallyt, a 400 Bad Request error was encountered while trying to use an ErrorDocument to handle the request.

Apache configuration:

ErrorLog /var/log/apache.log
LogLevel notice

ErrorDocument 400 /errHandler

Any ideas?