XE10 Datasnap REST Server ingore null string parameter and shift value

Hi all,

This example is for demostration error.
In the datasnap REST Server, declared this function :

TServerMethods1 = class(TDataModule)
private
public
function EchoString(A,B: string; const C: Integer): string;
end;

......

function TServerMethods1.EchoString(A,B: string; const C: Integer): string;
begin
if c= 1 then
Result := a + b
else
Result := 'b'
end;

In the client application generate the client rest code by IDE and call the function:
procedure TForm3.Button1Click(Sender: TObject);
begin
ClientModule1.ServerMethods1Client.EchoString('TEST','',10);
end;

Into the server function. received this:
A = 'TEST'
B= '10' <------- This is the C parameters
C=0

If i call :ClientModule1.ServerMethods1Client.EchoString('TEST','HALLO',10);
Work fine.
How to pass null or '' string paramaters?

This example working in XE7 and XE8, only XE10 has this effect.

Why?

thx
Roby