dcl-Control-SetVisible / NIL value not allowed

At the moment I'm rather confused.

I have a "chain" of modal forms, the last shows a lot of pictures and has two buttons "Show part 1" and "Show part 2. The first image has also a click-event.

I call the form, it is displayed, the test-onclick-event on first image is triggered. Then I press the button "Show part 2" and this routine is called:

(defun c:Einstellung/dlg_schraff/btn_teil1#OnClicked (/ zaehler)

    (alert "hallo1")

    (dcl-Control-SetVisible rbi_siaEinstellung/dlg_schraff/PictureBox1 t)

    (dcl-Control-SetVisible rbi_siaEinstellung/dlg_schraff/PictureBox2 t)

    (dcl-Control-SetVisible rbi_siaEinstellung/dlg_schraff/PictureBox3 t)

    (dcl-Control-SetVisible rbi_siaEinstellung/dlg_schraff/PictureBox4 nil)

    (dcl-Control-SetVisible rbi_siaEinstellung/dlg_schraff/PictureBox5 nil)

)

The code starts, I get the alert, and then I get

Error: NIL value not allowed

Function: dcl-control-Setvisible

Argument: 2

Any hints .. ??