Problem when error message in exit Save document prepare(MV50AFZ1) on PGI

Hi,

I am using USER_EXIT_DOCUMENT_PREPARE for throwing error message during PGI. The error triggers properly, but after that if I press enter button, the program ends with an ABEND error "Risk of posting several mat.documents for one delivery->long text".

In the EXIT code no change any data, only check, and when I comment exit code the PGI finish without problems.

when press save and not PGI, message error finish OK.

I have SAP 460c.

Thanks in Adv.

Regards

Matías