Invalid command Header

On a web site running Apache, i got following error in log file

[Fri Jan 19 06:43:41.146643 2018] [core:alert] [pid 3536:tid 139697024132864] [client] /home/ Invalid command 'Header', perhaps misspelled or defined by a module not included in the server configuration, referer:

This was Ubuntu server with Apache.

Problem is due to Apache Headers module not enabled. Fixed by running

a2enmod headers

Restart Apache

systemctl restart apache2

See Apache