at com.example.service.unit_testing.GeminiClientServiceTest.generateContent_whenApiResponseIsNull_throwsGeminiClientException(GeminiClientServiceTest .java:72) -> at com.example.service.unit_testing.GeminiClientServiceTest.generateContent_whenApiResponseIsNull_throwsGeminiClientException(GeminiClientServiceTest .java:72) -> at com.example.service.unit_testing.GeminiClientServiceTest.generateContent_whenApiResponseIsNull_throwsGeminiClientException(GeminiClientServiceTest .java:72) You cannot use argument matchers outside of verification or stubbing. Examples of correct usage of argument matchers: when(mock.get(anyInt())).thenReturn(null); doThrow(new RuntimeException()).when(mock).someVoidMethod(any()); verify(mock).someMethod(contains("foo")) This message may appear after an NullPointerException if the last matcher is returning an object like any() but the stubbed method signature expect a primitive argument, in this case, use primitive alternatives. when(mock.get(any())); // bad use, will raise NPE when(mock.get(anyInt())); // correct usage use Also, this error might show up because you use argument matchers with methods that cannot be mocked. Following methods *cannot* be stubbed/verified: final/private/equals()/hashCode(). Mocking methods declared on non-public parent classes is not supported. at com.example.service.unit_testing.QuickReplyContentServiceTest.setUp(QuickReplyContentServiceTest.java:49) at java.base/java.lang.reflect.Method.invoke(Method.java:580) at java.base/java.util.ArrayList.forEach(ArrayList.java:1596) at java.base/java.util.ArrayList.forEach(ArrayList.java:1596) ]]>