MethodAddress - How do I call the Method found

I have a class containing three procedures, each of which
expect a string as a parameter.  I can find the address
of the procedure to call by using MethodAddress.  However,
when I call the procedure using the address returned, and
pass a string, the value of the string is being passed as
empty to the procedure.

How do I overcome this?  I have been through the
documentation and source without luck.

The code below is a test of MethodAddress I have created
as an example.  The user enters a string in a text box and presses a button.  The first char entered is the procedure
to be executed, and the remaining characters are passed to
the procedure.

<SNIP - uses & form def>

  TGenericMethod = procedure(StringIn : string);

  TProcedures = class(TObject)
    procedure A(StringIn : string);
    procedure B(StringIn : string); virtual;
    procedure C(StringIn : string);
  end;

var
  Form1: TForm1;
  Procedures : TProcedures;

implementation

{$R *.DFM}

procedure TProcedures.A(StringIn : string);
begin
  MessageDlg('Procedure A - ' + StringIn, mtInformation, [mbOK], 0);
end;

procedure TProcedures.B(StringIn : string);
begin
  MessageDlg('Procedure B - ' + StringIn, mtInformation, [mbOK], 0);
end;

procedure TProcedures.C(StringIn : string);
begin
  MessageDlg('Procedure C - ' + StringIn, mtInformation, [mbOK], 0);
end;

procedure TForm1.btnRunProcClick(Sender: TObject);
var
  CallMethod : TGenericMethod;
  StringPar : string;
begin
  Procedures := TProcedures.Create;
  CallMethod := Procedures.MethodAddress(Copy(edtProcName.Text,1,1));
  StringPar  := Copy(edtProcName.Text,2,Length(edtProcName.Text) - 1);
  if Assigned(CallMethod) then
    CallMethod(StringPar)
  else
    MessageDlg('Procedure doesn''t exist', mtError, [mbOK], 0);
  Procedures.Free;
end;