Mockito Exception - when（） 需要一个参数，该参数必须是 mock 上的方法调用

我有一个非常简单的测试用例，它使用Mockito和Spring Test框架。当我这样做

when(pcUserService.read("1")).thenReturn(pcUser);

我得到这个例外。

org.mockito.exceptions.misusing.MissingMethodInvocationException: 
when() requires an argument which has to be 'a method call on a mock'.
For example:
    when(mock.getArticles()).thenReturn(articles);

Also, this error might show up because:
1. you stub either of: final/private/equals()/hashCode() methods.
   Those methods *cannot* be stubbed/verified.
2. inside when() you don't call method on mock but on some other object.

    at com.project.cleaner.controller.test.PcUserControllerTest.shouldGetPcUser(PcUserControllerTest.java:93)
    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 org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)

我尝试了不同的方法，但不断收到此错误消息。我正在使用Spring 3.1.0.RELEASE和Mockito。请分享并指导我朝着正确的方向前进。