PVS-Studio warning V568 for FCEUX

BOOL updateResults(HWND hwndDlg, int rule)
{
  char buff[0x100];
  ...
  SendDlgItemMessage(hwndDlg, RULE_INPUT_1 + i, WM_GETTEXT,
                     sizeof(buff - 1), (LPARAM) input_buff);
  ...
}

This is what should have been written here: sizeof(buff) - 1.

This suspicious code was found in FCEUX project by PVS-Studio static code analyzer.
Warning message is:
V568 It's odd that the argument of sizeof() operator is the 'buff - 1' expression. fceux monitor.cpp 117

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