REQUIRED_FIELD_MISSING error syncing: Upsert to Salesforce (WordPress user with ID of 18)

Object: Contact

Message: Required fields are missing: [LastName]

Params: Array
(
[Email] => yoshi_anko@hotmail.com.uat
[FirstName] =>
[LastName] =>
)

Upsert key: Email

Upsert value: yoshi_anko%40hotmail%2Ecom%2Euat