REQUIRED_FIELD_MISSING error syncing: Upsert to Salesforce (WordPress user with ID of 5)

Object: Contact

Message: Required fields are missing: [LastName]

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

Upsert key: Email

Upsert value: tom%2Etest%40growndev%2Epl