co11n check on field

hi experts,

I am using WORKORDER_GOODSMVT BADI to put check on quantity field of CO11n tcode. I found the method GOODS_RECEIPT having the changing parameter CT_GOODS_RECEIPT which is containing the field ERFMG (quantity).

I created an implementation ZBADI_co11n for the BADI.

I am trying to write code like this :

IF ct_goods_receipt-erfmg = 0.

MESSAGE e001(0) WITH ' Please Enter Exact Quantity ' .

ENDIF.

it is throwing error CT_GOODS_RECEIPT is a table without header line therefore has no component called ERFMG.

can anyone tell what to write in the mehtod GOODS_RECEIPT for my check on field.

help pls