Problem with LPBYTE lpBuffer - I always get the variable error - <bad ptr>

Hello all!

I've got a problem with the variable LPBYTE lpBuffer - Debug watch
reports me that this variable has a bad ptr. Then it's the cause that
I'm not able to get success with loading my file which I need to see in
my dialog's edit-box.

This is the problem part of my code:
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    HANDLE hFile;
    DWORD dwSize;
    DWORD dw;
    LPBYTE lpBuffer = NULL;
    lstrcpy(szSoubor, TEXT("\\temp\\test.txt"));
    hFile = CreateFile(szSoubor, GENERIC_READ, 0, NULL, OPEN_EXISTING, 0,
NULL);
    if(hFile == INVALID_HANDLE_VALUE)
        return;
    dwSize = GetFileSize(hFile, NULL);
    lpBuffer = (LPBYTE)HeapAlloc(GetProcessHeap(),
HEAP_GENERATE_EXCEPTIONS, dwSize+1);
    if(!ReadFile(hFile, lpBuffer, dwSize, &dw, NULL)){
        CloseHandle(hFile);
        HeapFree(GetProcessHeap(), 0, lpBuffer);
        return;
    }
    CloseHandle(hFile);
    lpBuffer[dwSize] = 0;
    SetDlgItemText(hwndDlg, IDC_EDIT1, (LPCTSTR)lpBuffer);
    SetFocus(hwndDlg);
    return;
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Did I do something wrong or did I forget something?
(I need the command <if(!ReadFile(...))> to get nonzero result.)

Thank you very much for any help!