JAVA: аргумент JUNIT, переданный в when(), не является макетом

Я пытаюсь запустить модульный тест для функции ниже

    @Autowired
    private EmailServiceConfigProperties emailServiceConfigProperties;
    
    @Autowired
    private HttpHandler httpHandler;
        
    public TransactionalEmailResponse sendPostRequestForTransactionalEmail(String endpoint, TransactionalEmail emailRequestBody) {
        HttpHeaders httpHeaders = setHttpHeaders(); 
        ResponseEntity<TransactionalEmailResponse> response = null;
        try {
            response = httpHandler.sendPost(endpoint, httpHeaders, emailRequestBody, TransactionalEmailResponse.class);
        } catch (Exception e) {
            logger.error("An error while sending POST request to email-service : " + JsonUtils.jsonize(emailRequestBody), e);
        }
        TransactionalEmailResponse body = response.getBody();
        return body;
    }

Ниже приведен модульный тест того же самого:

@InjectMocks
    private APIKeyAuthHandler testAPIKeyAuthHandler;
    
    @Mock
    private HttpHandler httpHandler;
    
    @Mock
    private EmailServiceConfigProperties emailServiceConfigPropertiesMock;

@Test
    public void testSendPostRequestForTransactionalEmail() {
        TransactionalEmail transactionalEmail = getTransactionalEmailRequestBody(); //returns POJO
        TransactionalEmailResponse transactionalEmailResponse = transcationEmailResponse();
        
        Mockito.doReturn(setHttpHeaders()).when(testAPIKeyAuthHandler).setHttpHeaders(); returns POJO
        String sendEmailEndpoint = "https://dev.com/api/v2/send/email";
        Mockito.when(httpHandler.sendPost(sendEmailEndpoint, setHttpHeaders(), transactionalEmail, TransactionalEmailResponse.class))
        .thenReturn(new ResponseEntity<>(transactionalEmailResponse, HttpStatus.OK));
        
        assertNotNull(testAPIKeyAuthHandler.sendPostRequestForTransactionalEmail(sendEmailEndpoint, transactionalEmail));
    }

Ниже приведен лог ошибки:

org.mockito.exceptions.misusing.NotAMockException: 
Argument passed to when() is not a mock!
Example of correct stubbing:
    doThrow(new RuntimeException()).when(mock).someMethod();
    at service.api.handlers.APIKeyAuthHandlerUnitTest.testSendPostRequestForTransactionalEmail(APIKeyAuthHandlerUnitTest.java:42)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:725)
    at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain$ValidatingInvocation.proceed(InvocationInterceptorChain.java:131)
    at org.junit.jupiter.engine.extension.TimeoutExtension.intercept(TimeoutExtension.java:149)
    at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestableMethod(TimeoutExtension.java:140)
    at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestMethod(TimeoutExtension.java:84)
    at org.junit.jupiter.engine.execution.ExecutableInvoker$ReflectiveInterceptorCall.lambda$ofVoidMethod$0(ExecutableInvoker.java:115)
    at org.junit.jupiter.engine.execution.ExecutableInvoker.lambda$invoke$0(ExecutableInvoker.java:105)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain$InterceptedInvocation.proceed(InvocationInterceptorChain.java:106)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain.proceed(InvocationInterceptorChain.java:64)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain.chainAndInvoke(InvocationInterceptorChain.java:45)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain.invoke(InvocationInterceptorChain.java:37)

Может ли кто-нибудь помочь мне с тем, как исправить то же самое? Спасибо