REQUIRED_FIELD_MISSING error syncing: Upsert to Salesforce (WordPress user with ID of 6)

Object: Contact

Message: Required fields are missing: [LastName]

Params: Array
(
[Email] => justine@noblecx.com.au
[FirstName] =>
[LastName] =>
)

Upsert key: Email

Upsert value: justine%40noblecx%2Ecom%2Eau