Disabled close button enables when moving the dialog

Question

So i want to disable [X] close button of MFC dialog while some process is running. I do it like this:

void CSomeDlg::EnableCloseButton(BOOL bEnable)
{
    UINT menuf = bEnable ? (MF_BYCOMMAND) : (MF_BYCOMMAND | MF_GRAYED | MF_DISABLED);

    CMenu* pSM = GetSystemMenu(FALSE);
    if(pSM)
        pSM->EnableMenuItem(SC_CLOSE, menuf);
}

void CSomeDlg::OnBtnClick_Cancel()
{
    if (!isSomeProcess)     
        CDialogEx::OnCancel();

}

void CSomeDlg::main()
{
    StartSomeProcess();
    isSomeProcess = true;
    EnableCloseButton(FALSE);

    //...

    EndSomeProcess();
    isSomeProcess = false;
    EnableCloseButton(TRUE);
}   

This works. Close button disables while some process is running. But if i move the dialog by dragging it with mouse the close button for some reason enables. And when i click on it application throws the crash error but it still working. And if i would't push the "Close application" button in the error dialog the application will finish the work correctly and error dialog will dissapear.

Why does close button enables? How to prevent it?