API is returning code 500 all the time

Question

My task is to send data to API(apiary) using POST. I am using retrofit and when I do post API is returning code 500.

My Retrofit client method looks like this:

@POST("api/employees")
Call<Employee> createEmployee(@Body Employee e);

I am making call like this:

EmployeeClient client = retrofit.create(EmployeeClient.class);
    Call<Employee> call = client.createEmployee(employee);

    call.enqueue(new Callback<Employee>() {
        @Override
        public void onResponse(Call<Employee> call, Response<Employee> response) {
            Toast.makeText(CreateEmployeeActivity.this, "code" + response.code(), Toast.LENGTH_SHORT).show();
        }

        @Override
        public void onFailure(Call<Employee> call, Throwable t) {
            Toast.makeText(CreateEmployeeActivity.this, "Something wrong", Toast.LENGTH_SHORT).show();
        }
    });

When I look to apiary inspector expected value for POST is something like this:

+ Part (name="json")
{
    "name": "John",
    "department": "IT",
    "address": "Somewhere",
    "photoUrl": "http://www.example.com/tesing/employees/65/photo"
}
+ Part (name="photo"; filename="filename.jpg")

If I look in POST description the photoUrl and photo is optional so in my POST I am sending Employee without the Photo file.