According to the GraphQL specifications (Oct.

According to the GraphQL specifications (Oct. 2016):
“For the sake of simplicity nullable types are always optional and non‐null types are always required.”
In your example, you have to make the `firstName` field nullable in your mutation and to reject a null value for this field in your code at the runtime.
This is a workaround in the waiting for a new GraphQL syntax to define a non-null AND optional field in a mutation input.