Inno Setup TInputDirWizardPage to not allow UNC path

I have a TInputDirWizardPage to allow the user to pick a backup directory. It automatically allows and error checks the entered paths including UNC paths. The backup directory cannot be a UNC path. How can I make it not allow and error check UNC paths?

procedure InitializeWizard();
begin
  BackupInfoPage :开发者_运维问答= CreateInputDirPage(100, 'caption', 'desc', 'sub caption', False,'Backup');
  BackupInfoPage.Add('Backup Location:');
  with BackupInfoPage do
  begin
      OnNextButtonClick := @BackupInfoForm_NextButtonClick;
  end;
end;

function BackupInfoForm_NextButtonClick(Page: TWizardPage): Boolean;
begin
  if not DirExists(BackupInfoPage.Values[0]) then
  begin
    ForceDirectories(BackupInfoPage.Values[0]);
  end;
end;

I did this, but it didn't seem to do anything.

[Setup]
AllowUNCPath=false

So I added a check on the NextButtonClick to keep them on the enter path page until they enter a valid, non-unc path.

if (Copy(BackupLocale, 1, 2) = '\\') then
  begin
    MsgBox('UNC paths are not allowed.', mbError, MB_OK);
    Result := False;
  end;