CharInSet function is not working in Delphi using TMS WEB Core

Question

I've written the following code on a button click event:

procedure TForm1.WebButton1Click(Sender: TObject);
var
  c: Char;
begin
  c := 'A';
  if CharInSet(c, ['0'..'9']) then
    ShowMessage('The character is a digit')
  else
    ShowMessage('The character is not a digit');
end;

But this gives a weird error:

[Error] Unit1.pas(31): not yet implemented: :TBinaryExpr [20170714171218]

I'm not sure if the error is on the CharInSet function or on the ['0'..'9'], but the error is definitely happening on that line with the CharInSet.

It seems like CharInSet either isn't supported in TMS WEB Core, or the way I'm using it isn't supported in TMS WEB Core.

How can I use CharInSet in TMS WEB Core?