Delphi "case of" expression expects constant

Question

I was porting my communication library written in C to Delphi which uses #define and "switch case" for one of the function.

I have translated them by using "const" and "case of" in Delphi.

But Delphi compiler complains by saying that "[dcc32 Error] Comm.Meter.pas(76): E2026 Constant expression expected".

As long as I see, there is nothing non-constant. Why is this? And what is the proper solution?

Const
  COM_SOH: Byte = 1;    // Start of Header
  COM_STX: Byte = 2;    // Start of Text
  COM_ETX: Byte = 3;    // End of Text
  COM_ACK: Byte = 6;    // Acknowledge
  COM_NACK: Byte = 21;  // Not acknowledge
  COM_CR: Byte = 13;
  COM_LF: Byte = 10;
  COM_DIV: Byte = 47;

function test_case(const ch: byte): Int32;
begin
  Result := 0;

  case ch of
    COM_DIV:
    begin
      Result := ch + 1;
    end;

    COM_ACK:
    begin
      Result := ch + 2;
    end;

    COM_NACK:
    begin
      Result := ch + 3;
    end;

    COM_STX:
    begin
      Result := ch + 4;
    end;
  end;
end;