Mockito的参数在Kotlin中返回null

给定以下类(用kotlin编写):

class Target {
     fun <R> target(filter: String, mapper: (String) -> R): R = mapper(filter)
}

我可以用java进行测试,测试代码:

@Test
public void testInJava() {
    Target mockTarget = Mockito.mock(Target.class);
    Mockito.when(mockTarget.target(
            argThat(it -> true),
            Mockito.argThat(it -> true)
    )).thenReturn(100);
    assert mockTarget.target("Hello World", it -> 1) == 100;
}

java测试按预期通过,但相同的测试是用kotlin编写的:

@Test
fun test() {
    val mockTarget = Mockito.mock(Target::class.java)
    Mockito.`when`(mockTarget.target(
            Mockito.argThat<String> { true },
            mapper = Mockito.argThat<Function1<String, Int>>({ true }))
    ).thenReturn(100)
    assert(mockTarget.target("Hello World") { 1 } == 100)
}

kotlin版本收到以下异常:

java.lang.IllegalStateException: Mockito.argThat<String> { true } must not be null

为什么会发生这种情况,我如何使用kotlin进行测试?