Strange Mockito test framework behaviour on Android

I am starting writing tests for my app and currently encountering very strange behaviour with Mockito framework. Basically I have simple following test class:

    package com.app.greatapp;

    import org.junit.Before;
    import org.junit.Test;
    import org.junit.runner.RunWith;
    import org.mockito.Mock;
    import org.mockito.runners.MockitoJUnitRunner;

    import static junit.framework.Assert.assertTrue;
    import static org.mockito.Mockito.when;

    @RunWith(MockitoJUnitRunner.class)
    public class DateToolsTests {

    @Mock Context context;

    @Before
    public void setUp() {
        when(context.getString(R.string.app_name)).thenReturn(" app");
    }

    @Test
    public void makeMyTest() {
        assertTrue(context.getString(R.string.app_name).equals(" app"));
    }
}

I am using class above (with totally same code) in two projects (and in the same PC) for testing purposes. In one project test passes fine but in another I get such exception:

java.lang.RuntimeException: Stub!

    at android.content.Context.getString(Context.java:14)
    at com.app.funguide.DateToolsTests.setUp(DateToolsTests.java:21)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:24)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.mockito.internal.runners.JUnit45AndHigherRunnerImpl.run(JUnit45AndHigherRunnerImpl.java:37)
    at org.mockito.runners.MockitoJUnitRunner.run(MockitoJUnitRunner.java:62)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:119)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:42)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:234)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:74)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

Seems that mockito can't mock Context object for some strange reasons...

In both projects build.gradle files I have following dependency:

testCompile 'org.mockito:mockito-core:1.10.19'

And using newest gradle plugin version:

classpath 'com.android.tools.build:gradle:2.2.3'

Anybody have ideas? It looks really basic code according to documentation, tutorials etc. and it's very strange why it is working in one project and in other not.