Add fields to a Groovy class using FieldNode

Question

Is it possible to create a Groovy class dynamically and add fields to it using the FieldNode class? Here is my attempt:

package asttest


import org.codehaus.groovy.control.*
import org.codehaus.groovy.ast.*
import org.codehaus.groovy.ast.stmt.*
import org.codehaus.groovy.ast.expr.*


GroovyClassLoader loader = new GroovyClassLoader()

cls = loader.parseClass('class Position{}')

ClassNode integerClassNode = new ClassNode(Integer.class)

ClassNode classNode = new ClassNode(cls)

FieldNode xFieldNode = new FieldNode(
    'x', FieldNode.ACC_PUBLIC, integerClassNode, classNode, new EmptyExpression())
FieldNode yFieldNode = new FieldNode(
    'y', FieldNode.ACC_PUBLIC, integerClassNode, classNode, new EmptyExpression())
FieldNode zFieldNode = new FieldNode(
    'z', FieldNode.ACC_PUBLIC, integerClassNode, classNode, new EmptyExpression())

classNode.addField(xFieldNode)
classNode.addField(yFieldNode)
classNode.addField(zFieldNode)

CompilationUnit compilationUnit = new CompilationUnit(loader)
compilationUnit.addClassNode(classNode)

compilationUnit.compile()

instance = cls.newInstance()
instance.x = 1

The result when running this code is:

Caught: BUG! exception in phase 'class generation' in source unit '?' unexpected NullpointerException
BUG! exception in phase 'class generation' in source unit '?' unexpected NullpointerException
    at asttest.Main.run(Main.groovy:35)
Caused by: java.lang.NullPointerException
    ... 1 more

I know I can add fields to a class using MetaClass (I posted a separate question), but it seems that using Groovy AST classes would be more flexible.