MockitoException while trying to test a servlet

I am trying to test a simple log in servlet that I have created with Mockito. I've been doing a bit of research on it, and I feel as though what I have written should work, but I am getting an "org.mockito.exceptions.base.MockitoException"

Here is my test class:

@RunWith(GwtMockitoTestRunner.class)
public class LoginServletTest {
 @Test
  public void testLogInServlet() throws Exception {
  final HttpServletRequest request = Mockito.mock(HttpServletRequest.class);
  final HttpServletResponse response = 
  Mockito.mock(HttpServletResponse.class);
  when(request.getParameter("username")).thenReturn("testuser");
  when(request.getParameter("password")).thenReturn("testpass");

  final StringWriter stringWriter = new StringWriter();
  final PrintWriter writer = new PrintWriter(stringWriter);
  when(response.getWriter()).thenReturn(writer);

  new LogInServlet().doPost(request, response);

  assertTrue(stringWriter.toString().contains("My expected string"));
 }

I am getting this exception:

org.mockito.exceptions.base.MockitoException: 
ClassCastException occurred while creating the mockito proxy :
  class to mock : 'javax.servlet.http.HttpServletRequest', loaded by classloader : 'sun.misc.Launcher$AppClassLoader@18b4aac2'
  created class : '$javax.servlet.http.HttpServletRequest$$EnhancerByMockitoWithCGLIB$$d31fd980', loaded by classloader : 'org.mockito.internal.creation.util.SearchingClassLoader@fbd1f6'
  proxy instance class : null
  instance creation by : ObjenesisInstantiator

You might experience classloading issues, disabling the Objenesis cache *might* help (see MockitoConfiguration)
    at com.mypackage.mywidget.mywidget.LoginServletTest.<init>(ReportingWidgetMockTest.java:50)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at org.junit.runners.BlockJUnit4ClassRunner.createTest(BlockJUnit4ClassRunner.java:195)
    at org.junit.runners.BlockJUnit4ClassRunner$1.runReflectiveCall(BlockJUnit4ClassRunner.java:244)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.BlockJUnit4ClassRunner.methodBlock(BlockJUnit4ClassRunner.java:241)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
    at com.google.gwtmockito.GwtMockitoTestRunner.run(GwtMockitoTestRunner.java:373)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:678)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)
Caused by: java.lang.ClassCastException: $javax.servlet.http.HttpServletRequest$$EnhancerByMockitoWithCGLIB$$d31fd980 cannot be cast to org.mockito.cglib.proxy.Factory
    at org.mockito.internal.creation.cglib.ClassImposterizer.createProxy(ClassImposterizer.java:143)
    at org.mockito.internal.creation.cglib.ClassImposterizer.imposterise(ClassImposterizer.java:58)
    at org.mockito.internal.creation.cglib.ClassImposterizer.imposterise(ClassImposterizer.java:49)
    at org.mockito.internal.creation.cglib.CglibMockMaker.createMock(CglibMockMaker.java:24)
    at org.mockito.internal.util.MockUtil.createMock(MockUtil.java:33)
    at org.mockito.internal.MockitoCore.mock(MockitoCore.java:59)
    at org.mockito.Mockito.mock(Mockito.java:1285)
    at org.mockito.Mockito.mock(Mockito.java:1163)
    ... 24 more

The line that causes this error is:

final HttpServletRequest request = Mockito.mock(HttpServletRequest.class);

I am new to mockito, but from what I gather this error mean that mockito is having trouble mocking the HttpServletRequest. I am not sure why or what I can do to fix this. Any help would be appreciated.