Failed to transform received changeset: Schema mismatch: Property in class is nullable on one side and not on the other

I am trying to set up an android app with realm, but I get this error: Failed to transform received changeset: Schema mismatch: Property 'firstName' in class 'Customer' is nullable on one side and not on the other. Both in the android app and in the realm app it is a string.

{
  "title": "Customer",
  "properties": {
    "_id": {
      "bsonType": "objectId"
    },
    "_partition": {
      "bsonType": "string"
    },
    "email": {
      "bsonType": "string"
    },
    "firstName": {
      "bsonType": "string"
    },
    "lastName": {
      "bsonType": "string"
    },
    "phone": {
      "bsonType": "string"
    }
  }
}

public class Customer extends RealmObject {
    @PrimaryKey
    private ObjectId _id = new ObjectId();

    private String _partition;
    @Required
    private String firstName;
    @Required
    private String lastName;

    private String phone;
    @Required
    private String email;
   
    //getters, setters, ctor..
}

I can't see what I did wrong, please help.