VerifyError using JFreeChart and Java 6

I am trying to unit test an old legacy application that uses JFreeChart and I'm trying to create a spy of a JFreeChart object this way:

JFreeChart chart = PowerMockito.spy(ChartFactory.createTimeSeriesChart("Mocked Name", "Date", "Mocked YAxis Name", dataset, true, true, false));

I don't know if it is even valid to create a spy this way, I've only ever done it before with the new operator. But I did try just making a mock like this and got the same error:

JFreeChart chart = Mockito.mock(JFreeChart.class);

Here is the error I'm getting:

java.lang.VerifyError: (class: javax/swing/plaf/metal/MetalLookAndFeel, method: getLayoutStyle signature: ()Ljavax/swing/LayoutStyle;) Wrong return type in function
at javax.swing.UIManager.setLookAndFeel(UIManager.java:554)
at javax.swing.UIManager.initializeDefaultLAF(UIManager.java:1317)
at javax.swing.UIManager.initialize(UIManager.java:1406)
at javax.swing.UIManager.maybeInitialize(UIManager.java:1394)
at javax.swing.UIManager.getDefaults(UIManager.java:633)
at javax.swing.UIManager.getColor(UIManager.java:675)
at org.jfree.chart.JFreeChart.<clinit>(JFreeChart.java:248)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:169)
at javassist.runtime.Desc.getClassObject(Desc.java:43)
at javassist.runtime.Desc.getClazz(Desc.java:52)
at org.jfree.chart.ChartFactory.createTimeSeriesChart(ChartFactory.java:1850)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.powermock.api.mockito.internal.invocation.MockitoMethodInvocationControl$1.invoke(MockitoMethodInvocationControl.java:242)
at org.powermock.api.mockito.internal.invocation.MockitoMethodInvocationControl.performIntercept(MockitoMethodInvocationControl.java:260)
at org.powermock.api.mockito.internal.invocation.MockitoMethodInvocationControl.invoke(MockitoMethodInvocationControl.java:192)
at org.powermock.core.MockGateway.doMethodCall(MockGateway.java:123)
at org.powermock.core.MockGateway.methodCall(MockGateway.java:62)
at org.jfree.chart.ChartFactory.createTimeSeriesChart(ChartFactory.java)
at co.ihc.com.heightweight.HeightWeightModuleTests.createChartTest(HeightWeightModuleTests.java:1558)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.junit.internal.runners.TestMethod.invoke(TestMethod.java:68)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.runTestMethod(PowerMockJUnit44RunnerDelegateImpl.java:310)
at org.junit.internal.runners.MethodRoadie$2.run(MethodRoadie.java:88)
at org.junit.internal.runners.MethodRoadie.runBeforesThenTestThenAfters(MethodRoadie.java:96)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.executeTest(PowerMockJUnit44RunnerDelegateImpl.java:294)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.runBeforesThenTestThenAfters(PowerMockJUnit44RunnerDelegateImpl.java:282)
at org.junit.internal.runners.MethodRoadie.runTest(MethodRoadie.java:86)
at org.junit.internal.runners.MethodRoadie.run(MethodRoadie.java:49)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl.invokeTestMethod(PowerMockJUnit44RunnerDelegateImpl.java:207)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl.runMethods(PowerMockJUnit44RunnerDelegateImpl.java:146)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$1.run(PowerMockJUnit44RunnerDelegateImpl.java:120)
at org.junit.internal.runners.ClassRoadie.runUnprotected(ClassRoadie.java:33)
at org.junit.internal.runners.ClassRoadie.runProtected(ClassRoadie.java:45)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl.run(PowerMockJUnit44RunnerDelegateImpl.java:122)
at org.powermock.modules.junit4.common.internal.impl.JUnit4TestSuiteChunkerImpl.run(JUnit4TestSuiteChunkerImpl.java:104)
at org.powermock.modules.junit4.common.internal.impl.AbstractCommonPowerMockRunner.run(AbstractCommonPowerMockRunner.java:53)
at org.powermock.modules.junit4.PowerMockRunner.run(PowerMockRunner.java:53)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

Having googled around a bit it looks like it could be due to a compatibility issue between my Java version and the Java version used to compile the jfreechart jar. I'm using Java 6 and here is my Maven dependency for jfreechart:

<dependency>
    <groupId>jfree</groupId>
    <artifactId>jfreechart</artifactId>
    <version>1.0.8a</version>
</dependency>

Any ideas of how to resolve this? Can I just use a different version of jfreechart and if so, does anyone know which version?