Schema does not allow optional arguments

[vorpus]

I have a custom mutation with the signature
editPerson(personId: Int!, username: String, title: String, email: String): Person

When I attempt to call this

gql`
   mutation ($personId: Int!, $value: String!) {
    editPerson(personId: $personId, username: $value) {
       name
     }
   }`

It throws GraphQL error: Failed to invoke procedure apoc.cypher.doIt: Caused by: org.neo4j.exceptions.ParameterNotFoundException: Expected parameter(s): title, email

This should be allowed since only personId is required?