#21 "type unknown"

[Paul]

I used in a event-b model the following wittness
p = p_loc ↦ p_com ↦ p_sor ↦ p_por ↦ p_ava ↦ p_apr ↦ p_con ↦ p_cyl ↦ p_pocy ↦ (PERSONNE ∖ PERSONNE) ↦ ∅ ↦ ∅ ↦ ∅ ↦ ∅ ↦ ∅ ↦ ∅ ↦ ∅ ↦ dc0 ↦ FALSE ↦ ∅ ↦ ∅ ↦ FALSE ↦ ∅ ↦ FALSE ↦ ∅ ↦ FALSE ↦ ∅ ↦ ∅

when building the project the following error is showed : "type unknown". It concerns all the "∅" fields.
How to solve this problem ?
i try (PERSONNE ∖ PERSONNE) for once and it works bu is it the good way ?