error "non-concrete value struct" when trying to look on cue struct

[cueckoo]

Originally opened by @ying-jeanne in cuelang/cue#985

the cue version 0.3.2

When the inputdef.IncompleteKind() is cue.StructKind, i would like to loop on all fields of inputdef by using function iter, err := inputdef.Fields(cue.Optional(true), cue.ResolveReferences(true), cue.Concrete(false)).

Then I got an error panels.268435455: non-concrete value struct, knowing that i would like to loop on element not necessarily concrete. What potentially is wrong here?

[cueckoo]

Original reply by @myitcv in cuelang/cue#985 (comment)

Please can you share a repro of what you are seeing here?