partner function

Hi,

i want to create a partner roles in my vendor master record. in the partner function screen, i enter partner function cp which is contact person, then i enter "dycw_ven02" which is another vendor in the field call "number", then the system specify the error that "enter numeric number". could anyone the reason? does it mean i only can enter the vendor which was assigned with numeric number for partner role "cp"?

please help, will appreciate ur input.