Failed to generate optionalNonNullArg of enum type

Hi, I tried using optionalNonNullArg as shown here: https://spec.graphql.org/October2021/#sel-JALTFFTB6CABABSnoI with enum type but schemagen failed with

terminate called after throwing an instance of 'std::out_of_range'
  what():  map::at

Code sample to reproduce

enum optionalNonNullEnum {
	One
	Two
}

type optionalNonNullTest_1 {
	test(arg: optionalNonNullEnum! = One) : Boolean!
}

Same error goes for custom input type:

input optionalNonNullInput {
	testField: Int!
}

type optionalNonNullTest_2 {
	test(arg: optionalNonNullInput! = {testField: 1}) : Boolean
}

Boolean/Int/String works fine.

Start by reproducing the failure in schemagen with the enum and custom input examples from the issue, then compare them with the Boolean, Int, and String cases that work. Trace the handling of optional non-null default values and confirm done when both examples generate without the std::out_of_range exception.