REQUIRED_FIELD_MISSING error syncing: Upsert to Salesforce (WordPress user with ID of 19)

Object: Contact

Message: Required fields are missing: [LastName]

Params: Array
(
[Email] => test@grown.pl
[FirstName] =>
[LastName] =>
)

Upsert key: Email

Upsert value: test%40grown%2Epl