Prevent invalid chars in editbox

Hello, I have an editbox in a dialog where the user is only allowed to
enter integers. However, I cannot simply set the number flag in the
resource editor because the user must be able to enter negative integers
too. Therefore, I didn't create it with that flag. Instead I subclassed
CEdit and redefined OnKeyDown():

void
NumberEditBox::OnKeyDown(UINT nChar, UINT nRepCnt, UINT nFlags)
{
    static const std::string valid_chars = "0123456789-";

    if (valid_chars.find(nChar) != std::string::npos)
    {
       TRACE1("%c is a valid char\n", (char)nChar);
       CEdit::OnKeyDown(nChar, nRepCnt, nFlags);
    }
}

This is basically the only code in my NumberEditBox class, no other
member functions have been introduced or redefined by me.

When the program is run, I only see traces of the valid chars, however
the invalid ones still appear in the editbox itself. What else do I need
to do? I was suprised that not letting the CEdit do anything for the
unwanted chars was not enough.

I'm using MSVC++ 8.0 SP1 on WinXP Pro SP2.

- Eric