callback with non-buildin component error

code as following:
1 Private Sub objNotify_CallBackComplete(ByVal length As Long, info As
Byte)

2 Text1.Text = length
3 l = l + 1
4 If l > 30000 Then
5 l = 0
6 End If

7 Text2.Text = l
8 'RichTextBox1.TextRTF = l
9 putData l

10 End Sub

everything is ok if i replace the "richtextbox" with build in component as
text or list.
if i remark line 7 and make line 8 availbe, the system callback will be
started normally within the vb 6.0 IDE, and if i export it as a .exe file
error will occure during runtime.No error message but the callback halted at line 8.

if i use vb.net everything is ok.

help needed.
thx in advance.