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

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??