Groovy: #Immutable results in Setters - groovy

Question

I'm having trouble with (or I'm confused about) using #Immutability.
Given this class:
#Immutable
class TestField {
String key
}
I would expect I could not set a key on an instance like:
class TestFieldSpec extends Specification {
def 'do stuff'() {
when:
def t = new TestField('a')
println t
t.key = 'b'
println t
then:
t
}
}
However, when I run the test, I do not see a ReadOnlyPropertyException:
Running TestFieldSpec
TestField(a)
TestField(b)
This has been the case for me using both the groovy-eclipse-compiler and gmaven-plus with Maven.
Groovy version 2.4.7.
If I run javap on the class file created by both, I see:
public final void setKey(java.lang.String);
When I do the same in GroovyConsole though, things work how I'd expect them to.
#groovy.transform.Immutable
class TestField {
String key
}
def f = new TestField('a')
f.key = 'b'
Produces:
Exception thrown
groovy.lang.ReadOnlyPropertyException: Cannot set readonly property: key for class: TestField
at TestField.setProperty(ConsoleScript1)
at ConsoleScript1.run(ConsoleScript1:7)
In the Groovy AST Browser, at Phase Finalization, I do not see the setter.
Any insight would be greatly appreciated.
Thanks!