IdHTTPServer1CommandGet

Why in the point A property ARequestInfoContentLength = 38
but ARequestInfo.PostStream = nil  ???
How Can I extract content posted by a client?

procedure TForm1.IdHTTPServer1CommandGet(AContext: TIdContext;
                                         ARequestInfo: TIdHTTPRequestInfo;
                                         AResponseInfo:
TIdHTTPResponseInfo);
var
  aBuf: string;
begin
  SetLength( aBuf, ARequestInfo.PostStream.Size ); <=======A
  ARequestInfo.PostStream.Read( aBuf[1], ARequestInfo.PostStream.Size );