Dependent drop down - error while saving new rows

Encountering an error when attempting to save a new record. In this document type, the vendor name and number are linked fields. Upon selecting the vendor name from the dropdown, the corresponding phone number is auto-populated as expected. However, upon saving, an error is being displayed. The Vendor Name field is the primary key (PK) in the vendor table, and the Vendor Number is a phone number field in the vendor Doctype.