AssertionFailedError:java.lang.NullPointerExc

[Nobody/Anonymous]

When I start StrutsTest by running Junit in eclipase,it always says:

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junit.framework.AssertionFailedError: Error running action.perform(): class java.lang.NullPointerException - null
    at junit.framework.Assert.fail(Assert.java:47)
    at servletunit.struts.MockStrutsTestCase.actionPerform(MockStrutsTestCase.java:339)
    at classmate.TestLoginAction.testSuccessfulLogin(TestLoginAction.java:28)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at junit.framework.TestCase.runTest(TestCase.java:154)
    at junit.framework.TestCase.runBare(TestCase.java:127)
    at junit.framework.TestResult$1.protect(TestResult.java:106)
    at junit.framework.TestResult.runProtected(TestResult.java:124)
    at junit.framework.TestResult.run(TestResult.java:109)
    at junit.framework.TestCase.run(TestCase.java:118)
    at junit.framework.TestSuite.runTest(TestSuite.java:208)
    at junit.framework.TestSuite.run(TestSuite.java:203)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:478)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:344)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:196)
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I don't know what's wrong with it?

Thanks in advance for any help you can offer!