how to throw error if my property value is "NA" or "undefined" in ajv?

[lmk07]

Hi guys, i just want to know, how to throw error if my property value is "NA" or "undefined".

sample data:
                  `"data":{
                              "fname":"NA",
                              "lname":"undefined",
                              "age":"28" 
                    }`

now i want to show error for fname and lname property, what changes i need to make in schema?