FUNCTION MODULES

I have created a function module in which I have given the following things.

Export paramters:

s_erdat like vbak-erdat.

s_vkorg like vbak-erdat.

s_vtweg like vbak-vtweg.

s_bsark like vbak-bsark

s_spart like vbak-spart.

Tables:

itab like vbak.

Source Code:

SELECT vbeln bsark vtweg spart vdatu FROM vbak

INTO CORRESPONDING FIELDS OF TABLE itab

WHERE

erdat = s_erdat AND

vkorg = s_vkorg AND

vtweg = s_vtweg AND

bsark = s_bsark AND

spart = s_spart.

But when I am executing this Function Modules,the internal table shows zero entries.

Plz tell what to do.

Point will be rewarded nodoubt.