Dump GETWA_NOT_ASSIGNED

Hi guys.

I'm facing a short dump in an user exit during an invoice creation via BAPI_BILLINGDOC_CREATEMULTIPLE.

The statement is: ASSIGN ('(SAPLV60A)XVBRK[]') TO <fs_t_vbrk>.

At this point '(SAPLV60A)XVBRK[]' is not assigned.

How to solve it? Should I check if it is assigned before this statement?? How?

Thanks in advance,

Alm