How to apply directives on input object fields

[de55]

I have a field directive like this:

    override fun onField(environment: KotlinFieldDirectiveEnvironment): GraphQLFieldDefinition {
        val field = environment.element
        val originalDataFetcher: DataFetcher<*> = environment.getDataFetcher()
        val newFetcher = DataFetcher<Any> { dataEnv ->
            val fourDigitNumber: String? = dataEnv.getArgument(environment.element.arguments[0].name) as String?
            if (fourDigitNumber?.length != 4 || fourDigitNumber.toLongOrNull() == null) {
                throw Exception("Must only contain 4 numerical characters")
            }
            originalDataFetcher.get(dataEnv)
        }
        environment.setDataFetcher(newFetcher)
        return field
    }

This works great on arguments, but it does not work on an input object field. My first thought was to do the same thing overriding onInputObjectField, but the get/set datafetcher methods don't exist:

    override fun onInputObjectField(environment: KotlinSchemaDirectiveEnvironment<GraphQLInputObjectField>): GraphQLInputObjectField {
        val field = environment.element
        val originalDataFetcher: DataFetcher<*> = environment.getDataFetcher() //THIS METHOD DOESN'T EXIST
        val newFetcher = DataFetcher<Any> { dataEnv ->
            val fourDigitNumber: String? = dataEnv.getArgument(environment.element.arguments[0].name) as String?
            if (fourDigitNumber?.length != 4 || fourDigitNumber.toLongOrNull() == null) {
                throw Exception("Must only contain 4 numerical characters")
            }
            originalDataFetcher.get(dataEnv)
        }
        environment.setDataFetcher(newFetcher) //THIS METHOD DOESN'T EXIST
        return field
    }

Is there a way to do this on the fields of input objects? Or will this only work on arguments?