Search for unscheduled tasks

Is it possible to search for tasks that are not scheduled?
I know there exists :block/scheduled, but I only understand how to use it, if the task IS scheduled. What is the value, if it’s not?

The following doesn’t seem to work:

{:title "Inbox"
  :query [:find (pull ?h [*])
      [?h :block/scheduled ?d]
      [(= ?d "nil")]]}

This works for me:

 [:find (pull ?b [*])
[?b :block/marker ?marker]
(not [?b :block/scheduled ?sch])
[(contains? #{"LATER" "TODO"} ?marker)]]
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