says ambiguity where there is no ambiguity

Following fails:

A.java:
===
class A {
void f(boolean b) {
}
void f(String s) {
}
}
===

a.groovy
===
A.f(null);
===

Invoked as "groovy a.groovy" it fails with
===
Caught: groovy.lang.GroovyRuntimeException: failed to invoke method: org.codehaus.groovy.runtime.ReflectionMetaMethod@80f4cb[name: f params: [boolean] returns: void owner: class A] on: class A with arguments: [null] reason: java.lang.IllegalArgumentException
groovy.lang.GroovyRuntimeException: failed to invoke method: org.codehaus.groovy.runtime.ReflectionMetaMethod@80f4cb[name: f params: [boolean] returns: void owner: class A] on: class A with arguments: [null] reason: java.lang.IllegalArgumentException
at groovy.lang.MetaClass.doMethodInvoke(MetaClass.java:1075)
...
===

A.f((String) null) should have been called.