Michelson paires non comparables

Lorsquej'essaie detaper le script Michelson suivant,je reçois uneerreur detype.

 parameter unit;
storage (big_map (pair string address) (list timestamp));
code {CDR; NIL operation; PAIR};
 

 Ill formed type.
At line 2 characters 17 to 37,
comparable type expected.Type pair string address is not comparable.
 

Selon le Référence Michelson ,types pair ty1 ty2,addresset string sont comparables.Est-ce queje manque quelque chose?Est-ilpossible de contourner ceproblème?

When I try to type the following Michelson script, I get a type error.

parameter unit;
storage (big_map (pair string address) (list timestamp));
code {CDR; NIL operation; PAIR};

Ill formed type.
At line 2 characters 17 to 37,
comparable type expected.Type pair string address is not comparable.

According to the Michelson reference, types pair ty1 ty2, address, and string are comparable. Am I missing something? Is there are way to circumvent this issue?