systemd timer interval not being set correctly by install script

[MarkHedleyJones]

When trying to start the timer the following error occurs:

Failed to start dmenu-extended-update-db.timer: Unit dmenu-extended-update-db.timer has a bad unit file setting.
See user logs and 'systemctl --user status dmenu-extended-update-db.timer' for details.

Looking at the details shows

Failed to parse calendar specification, ig>
Jul 30 17:24:39 laptop systemd[7712]: dmenu-extended-update-db.timer: Timer unit lacks value setting. Refusing.

Looking at the timer value in the .timer file shows:

OnCalendar=*:0/None