unit test for REST-API #PutMapping controller - problem with JSON path - unit-testing

Question

I have problem with unit test for controller that has to update an employee in REST-API. For testing I use mockito, JUnit, hamcrest. I am only posting the most important parts of these classes.
controller method
#PutMapping("{id}")
public ResponseEntity<Employee> updateEmployee(#PathVariable(value = "id") Long id,
#RequestBody Employee employee) {
return new ResponseEntity<>(employeeService.UpdateEmployee(id, employee), HttpStatus.OK);
}
service methods
#Override
public Employee UpdateEmployee(Long employeeId, Employee employee) {
Employee employeeById = employeeRepository.findById(employeeId)
.orElseThrow(() -> new ResourceNotFoundException("Employee with id " + employeeId + " not found"));
employeeById.setFirstName(employee.getFirstName());
employeeById.setLastName(employee.getLastName());
employeeById.setEmail(employee.getEmail());
employeeRepository.save(employeeById);
return employeeById;
}
#Override
public Optional<Employee> getEmployeeById(Long id) {
return employeeRepository.findById(id);
}
unit test
#BeforeEach
public void setup() {
employee = Employee.builder()
.firstName(TEST_FIRST_NAME)
.lastName(TEST_LAST_NAME)
.email(TEST_EMAIL)
.build();
}
#Test
public void givenUpdatedEmployee_whenUpdateEmployee_thenReturnUpdateEmployeeObject() throws Exception {
//given
Long employeeId = 1L;
Employee employeeInfo = Employee.builder()
.firstName(NEW_TEST_FIRST_NAME)
.lastName(NEW_TEST_LAST_NAME)
.email(NEW_TEST_EMAIL)
.build();
BDDMockito.given(employeeService.getEmployeeById(employeeId))
.willReturn(Optional.of(employee));
BDDMockito.given(employeeService.UpdateEmployee(employeeId, employeeInfo))
.willAnswer((invocationOnMock -> invocationOnMock.getArgument(0)));
//when
ResultActions response = mockMvc.perform(put("/api/v1/employees/{id}", employeeId)
.contentType(MediaType.APPLICATION_JSON)
.content(objectMapper.writeValueAsBytes(employeeInfo)));
//then
response.andDo(print())
.andExpect(status().isOk())
.andExpect(jsonPath("$.firstName", is(employeeInfo.getFirstName())))
.andExpect(jsonPath("$.lastName", is(employeeInfo.getLastName())))
.andExpect(jsonPath("$.email", is(employeeInfo.getEmail())));
}
receives an error
java.lang.AssertionError: No value at JSON path "$.firstName"