argThat() causes TypeCastException

[StepanOvsyankin]

interface HelloSrv {
    fun sayHello(to: String): String
}

class AnyTest {
    @Test fun `should create non-null values`() {
        mock<HelloSrv> {
            on { sayHello(argThat { this == "A" }) } doReturn "A"
            on { sayHello(argThat { this == "B" }) } doReturn "B" // <--- fails here
        }
    }
}

fails with

kotlin.TypeCastException: null cannot be cast to non-null type kotlin.String

	at kkk.command.AnyTest$should create non-null values$mock$1$1$$special$$inlined$argThat$1.matches(Mockito.kt:55)
	at org.mockito.internal.invocation.TypeSafeMatching.apply(TypeSafeMatching.java:24)
	at org.mockito.internal.invocation.MatcherApplicationStrategy.forEachMatcherAndArgument(MatcherApplicationStrategy.java:82)
	at org.mockito.internal.invocation.InvocationMatcher.argumentsMatch(InvocationMatcher.java:152)
	at org.mockito.internal.invocation.InvocationMatcher.matches(InvocationMatcher.java:81)
	at org.mockito.internal.stubbing.InvocationContainerImpl.findAnswerFor(InvocationContainerImpl.java:82)
	at org.mockito.internal.handler.MockHandlerImpl.handle(MockHandlerImpl.java:88)
	at org.mockito.internal.handler.NullResultGuardian.handle(NullResultGuardian.java:32)
	at org.mockito.internal.handler.InvocationNotifierHandler.handle(InvocationNotifierHandler.java:36)
	at org.mockito.internal.creation.bytebuddy.MockMethodInterceptor.doIntercept(MockMethodInterceptor.java:41)
	at org.mockito.internal.creation.bytebuddy.MockMethodInterceptor$DispatcherDefaultingToRealMethod.interceptAbstract(MockMethodInterceptor.java:120)
	at kkk.command.HelloSrv$MockitoMock$934695312.sayHello(Unknown Source)
	at kkk.command.AnyTest$should create non-null values$mock$1$2.invoke(AnyTest.kt:16)
	at kkk.command.AnyTest$should create non-null values$mock$1$2.invoke(AnyTest.kt:12)
	at com.nhaarman.mockito_kotlin.KStubbing.on(Mockito.kt:119)
	at kkk.command.AnyTest.should create non-null values(AnyTest.kt:16)

mockito-kotlin: 1.0.0
mockito: 2.2.28

[nhaarman]

This happens because the null value returned by the second argThat is propagated by Mockito into the first argThat, which does not accept null values.

For now, you can workaround this by using the Java version like so:

mock<HelloSrv> {
    on { sayHello(Mockito.argThat { it == "A" } ?: createInstance()) } doReturn "A"
    on { sayHello(Mockito.argThat { it == "B" } ?: createInstance()) } doReturn "B"
}