Using javascript json class with ASP

I would like to persist ASP session data in JSON format. I have written this code :

Dim piece_ : Set piece_ = JSON.parse("{""code"":"""",""quantity"":""0"",""printed"":""false"",""familles"":""[]""}")
      piece_.set "code", Collection(key).code
      piece_.set "quantity", Collection(key).quantity
      piece_.set "familles", Collection(key).familles_.ToJavaScript()

I get an error with the Set method. My system is WinXp Pro SP3 - IIS 5. What's going wrong? Thx

Try piece_.set("code",Collection(key).code))