Unit testing Java class with Mockito mocks

I wrote 2 classes in Java. These are Developer and GroupManager. Developer has field private GroupManager supervisor (with getter and setter) describing who hires him. What is more, GroupManager has following method:

public class GroupManager {

    //...

    private List<Developer> employees;

    public void hire(Developer e) {
        if(canHire()) {
            if(employees.contains(e)) {
                System.out.println(e.getName() + " is already hired");
            }
            else {
                e.setSupervisor(this);
                employees.add(e);
                System.out.println(getName() + " is now hiring " + e.getName());
            }
        }
        else {
            System.out.println(getName() + "cannot hire more employees.");
        }
    }

}

This class exposes method hire which set supervisor on given developer. I would like to write tests for class Developer. I did it like this:

@Test
public void hiringWorksAsItShould() throws Exception {
    GroupManager gm = new GroupManager("John Doe", "manager", 1);
    gm.hire(developer);
    assertThat(developer.getSupervisor()).isInstanceOf(GroupManager.class);
}

I am using FEST testing framework but since it is very intuitive I belive everyone can understand what it means. This test passes, but because I want it to be unit test a decided to replace = new GroupManager with = mock(GroupManager.class). The problem is that now this test fails. How could I correct this Mockito feature to check is developer's supervisor is instance of GroupManager.

EDIT The error I get:

java.lang.AssertionError: expecting actual value not to be null
    at org.fest.assertions.Fail.failure(Fail.java:228)
    at org.fest.assertions.Fail.fail(Fail.java:167)
    at org.fest.assertions.Fail.failIfActualIsNull(Fail.java:100)
    at org.fest.assertions.GenericAssert.isNotNull(GenericAssert.java:238)
    at org.fest.assertions.ObjectAssert.isInstanceOf(ObjectAssert.java:52)
    at DeveloperTest.hiringWorksAsItShould(DeveloperTest.java:23)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    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.InvokeMethod.evaluate(InvokeMethod.java:17)
    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.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:78)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:212)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:68)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)

Thanks in advance.