[Pharo-project] [Q] help with requesting a html page

Friends: I have this code login | fields aPassword userName logged | fields := self request propertyAt: #postFields ifAbsent: []. fields ifNotNil:[ userName := (fields at: 'username'). aPassword := (fields at: 'password'). logged := HVNaughtieUsers new verifyPassword: aPassword forUser: userName ]. self halt. When I hit in the web browser , fields become nil. But if into Squeak I restart the method and do step by step, fields become populated with the right values. How I should do for the right values and avoid nil ? Thanks in advance Edgar