How do I set a sequence field at a specific number using url param

My schema has a field named "id" that is a sequence and I want to set the start number with a url param.

Route:
GET /Users.json/:start=100&step=1

Handler Script:
schema "Users"
set "id", start: params['start at'], step: params['step']
generate 10

Error : {"error":"undefined method `start=' for #\u003cColumn:0x007fd23baa0388\u003e"}

Any idea on what I'm doing wrong?