Ernie was also an excuse for Pterry to return to his theme (previously mentioned when describing the demon Urglefloggah in Eric) that calling something "a nightmare" is lazy and unrealistic since most nightmares are silly and don't sound horrific when described to another. Hence this exchange: Ernie: Who are you? Teatime: I'm glad you asked. I'm your worst nightmare. Ernie: Aww no! Not the one with the giant cabbage and the whirring knife thing! Teatime: Eh?
The dialogue is right, but it is not with Ernie, but with the "second guard" found in the Tower of Teeth (Corgi PB p. 100).