System Message 6934

Hi,

I  customized the possible values list for the vendor id field in the vendor maintenance screen so that it defaults to vendor name instead of vendor id.

This was successfully done.  However, on calling up the vendor id in the screen I keep getting a system message 6934 "attempting to set text control to an invalid value. Value violates the mask specification.

Could anyone point me in the correct direction to resolve this issue?  

Any  assistance will be greatly appreciated.

Thanks in advance