[arch-general] error: key "A6234074498E9CEE" could not be looked up remotely

[mick]

The last few days when I try to update (pacman -Syyu) I get the following:-

(29/29) checking keys in keyring                   [######################] 100%
downloading required keys...
error: key "A6234074498E9CEE" could not be looked up remotely
error: required key missing from keyring
error: failed to commit transaction (unexpected error)
Errors occurred, no packages were upgraded.

how do I fix it??