Problem with .isNil for string inside of macro

[Stefan_Salewski]

    proc t(i: int): string =
      if i != 0: return "OK"
    
    echo t(1)
    let x = t(0)
    echo x
    if x == nil: echo "is nil"
    if x.isNil: echo "is nil"
    if x == "": echo "is nil"
    
    macro mconnect(arg: typed): untyped =
      let sci = t(0)
      if sci.isNil:
        echo "nil1"
      if sci == nil:
        echo "nil2"
      if sci == "":
        echo "nil3"
    
    mconnect(0)
    
    
    Run
    
    
    $ nim c t.nim
    Hint: used config file '/home/stefan/Nim/config/nim.cfg' [Conf]
    Hint: system [Processing]
    Hint: t [Processing]
    nil2
    nil3
    Hint:  [Link]
    Hint: operation successful (12093 lines compiled; 0.131 sec total; 
16.324MiB peakmem; Debug Build) [SuccessX]
    
    
    Run

So there is no "nil1" output, indicating sci.isNil is not true as expected (by 
me).