undefined method `eager_autoload!' for Mail:Module (NoMethodError) on decko server start

Decko installed succesfully, all other dependencies seem to be in place and working correctly. It fails to start however with the error

undefined method `eager_autoload!' for Mail:Module (NoMethodError)

Rails 6.1.5

ruby 3.1.1p18 (2022-02-18 revision 53f5fc4236) [x86_64-linux]

(running in WSL)

Any help or pointers gratefully appreciated.