Unwise Child by Randall Garrett

Mike the Angel hefted the heavy stun gun in his right fist, feeling its weight without really noticing it. He knew damned good and well it wouldn't be of any use against Snookums. If Mellon came at him, the supersonic beam from the gun would affect his nerves the same way an electric current would, and he'd collapse, unconscious but relatively unharmed. But Mike doubted seriously that it would have any effect at all on the metal body of the robot. It is as difficult to jolt the nerves of a robot as it is to blind an oyster.

Snookums did have sensory devices that enabled him to tell what was going on around him, but they were not nerves in the ordinary sense of the word, and a stun gun certainly wouldn't have the same effect.