Unable to jsonify stringtable(s)

[random898989]

    import strtabs, json
    #let b:seq[StringTableRef] = @[]
    let b = newStringTable()
    echo %*{"a":10,"b":b}
    

fails with
    
    
    lib/pure/json.nim(760, 36) template/generic instantiation from here
    lib/pure/json.nim(772, 14) template/generic instantiation from here
    lib/pure/json.nim(765, 42) Error: undeclared field: 'counter'
    

any idea what's going on here? i'm on 0.17.2