[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