crypttab: location of luks header file

Hi my crypttab looks as follows: crypt_device /dev/sda luks,header=/boot/header.img update-initramfs -u -k all works with success, but for some reason cryptsetup will not find the header.img which resides on the usb stick (that also contains the boot partition) during boot. It is stored on /boot/header.img (using luks encryption with detached header, and seperate boot partition on usb, os: lubuntu 18)