// doesn't exist or is unusable, error.
if ( $target instanceof User &&
( $target->isAnon() /* doesn't exist */ || !User::isUsableName( $target->getName() ) )
// doesn't exist or is unusable, error.
if ( $target instanceof User &&
( $target->isAnon() /* doesn't exist */ || !User::isUsableName( $target->getName() ) )