Visual C++ :: Sending Unsigned Char To Listbox

Got something like the following. A button that read characters from a thrid party tool and sends it to a listbox. But the contant is not readable.

Code:
void dlg::sendtolistbox() {
unsigned char buf[250];
thirdparty.GetData(buf, len);
Sendmessage(hndl, listboxupdate,0 , (LPARAM)buf);
}
void Mydlg::UpdateListBox(wparam a,laparm b) {
m_listbox.AddString((LPCTSTR)b);
}

The characters show up in the list box as short unreadable characters. like it is chopped.

If i change to : Sendmessage(hndl, listboxupdate,0 , (LPARAM)&buf[15]);

Then I can see readable valid strings of up to 50 characters and then empty unreadable characters afterwards. I tried all kinds of things , including using CString, still did not work.