Kotlin and new ActivityTestRule : The @Rule must be public

I'm trying to make UI test for my android app in Kotlin. Since the new system using ActivityTestRule, I can't make it work: it compiles correctly, and at runtime, I get:

java.lang.Exception: The @Rule 'mActivityRule' must be public.
at org.junit.internal.runners.rules.RuleFieldValidator.addError(RuleFieldValidator.java:90)
at org.junit.internal.runners.rules.RuleFieldValidator.validatePublic(RuleFieldValidator.java:67)
at org.junit.internal.runners.rules.RuleFieldValidator.validateField(RuleFieldValidator.java:55)
at org.junit.internal.runners.rules.RuleFieldValidator.validate(RuleFieldValidator.java:50)
at org.junit.runners.BlockJUnit4ClassRunner.validateFields(BlockJUnit4ClassRunner.java:170)
at org.junit.runners.BlockJUnit4ClassRunner.collectInitializationErrors(BlockJUnit4ClassRunner.java:103)
at org.junit.runners.ParentRunner.validate(ParentRunner.java:344)
at org.junit.runners.ParentRunner.<init>(ParentRunner.java:74)
at org.junit.runners.BlockJUnit4ClassRunner.<init>(BlockJUnit4ClassRunner.java:55)
at android.support.test.internal.runner.junit4.AndroidJUnit4ClassRunner.<init>(AndroidJUnit4ClassRunner.java:38)
at android.support.test.runner.AndroidJUnit4.<init>(AndroidJUnit4.java:36)
at java.lang.reflect.Constructor.constructNative(Native Method)
at java.lang.reflect.Constructor.newInstance(Constructor.java:417)
at android.support.test.internal.runner.junit4.AndroidAnnotatedBuilder.buildAndroidRunner(AndroidAnnotatedBuilder.java:57)
at android.support.test.internal.runner.junit4.AndroidAnnotatedBuilder.runnerForClass(AndroidAnnotatedBuilder.java:45)
at org.junit.runners.model.RunnerBuilder.safeRunnerForClass(RunnerBuilder.java:57)
at org.junit.internal.builders.AllDefaultPossibilitiesBuilder.runnerForClass(AllDefaultPossibilitiesBuilder.java:29)
at org.junit.runner.Computer.getRunner(Computer.java:38)
at org.junit.runner.Computer$1.runnerForClass(Computer.java:29)
at org.junit.runners.model.RunnerBuilder.safeRunnerForClass(RunnerBuilder.java:57)
at org.junit.runners.model.RunnerBuilder.runners(RunnerBuilder.java:98)
at org.junit.runners.model.RunnerBuilder.runners(RunnerBuilder.java:84)
at org.junit.runners.Suite.<init>(Suite.java:79)
at org.junit.runner.Computer.getSuite(Computer.java:26)
at android.support.test.internal.runner.TestRequestBuilder.classes(TestRequestBuilder.java:691)
at android.support.test.internal.runner.TestRequestBuilder.build(TestRequestBuilder.java:654)
at android.support.test.runner.AndroidJUnitRunner.buildRequest(AndroidJUnitRunner.java:329)
at android.support.test.runner.AndroidJUnitRunner.onStart(AndroidJUnitRunner.java:226)
at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:1584)

Here is how I declared mActivityRule:

RunWith(javaClass<AndroidJUnit4>())
LargeTest
public class RadisTest {

    Rule
    public val mActivityRule: ActivityTestRule<MainActivity> = ActivityTestRule(javaClass<MainActivity>())

   ...
}

It is already public :/