Non primary key in where clause will not work??

Dear Experts,

I need to check the vendor is bidder ot not. I tried to check it by passing the Document Guid to CRMD_PARTNER table and by getting the partner guid and passing it to the table BUT000 i can check for partner function = 18.

But my problem is:

   select single partner_guid guid from CRMD_PARTNER
              into  wa_CRMD_PARTNER
              where GUID eq WA_CRMD_ORDERADM_H-lv_guid.

is not giving me the records i checked it in CRMD_PARTNER table the GUID field is not primary. In general for SAP select statements we cant use the non primary key with where clause?? Please suggest.

Thanks & Regards

Arun.K.P