Is anybody know how to use Var2Xml ?

When use with simple types - this work ok, but when I want use with my own 
dataset class i have en error..


24.03.2016;21:47:33: Zapis bledu : ERROR:Parameter has a wrong data type 
(<NIL> + </>)
Stos: MYBREAK(580) (B)DLL_PROC(324) DATASET:NOIVARCALLBACK(723) 
XMLMAPPER:FROMOBJECT(145) _VAR2XML(171) VAR2XML(186) PRINTDOKUMENT(5536) 
(B)HELPREJDOK(3195) REJESTRDOKUMENTOW(482) DLL_PROC(332) (B)GETTABMENU(0) 
RUNBLOCKSES(316) THREADRUN:EXECUTE(190)