Symfony Validation Error Message for Missing Required Form Field

I have a working Form, it has a required field that needs to be notBlank:

/**
 * @Assert\NotBlank
 */
private $field1 = '';

If I specify this field in the request, but leave the field empty, I get this response:

{
    "code":400,
    "message":"Validation Failed",
    "errors":{
        "children":{
            "field1":{
                "errors":["Field should not be blank"]
            }
        }
    }
}

If I omit this field from the request, I get this response:

{
    "code":400,
    "message":"Validation Failed",
    "errors":{
        "errors":["Field should not be blank"]
    }
}

Is there some built-in Symfony logic somewhere that I can use to make the second example match the first example?

[edit] Was using Symfony 2.5 - now updated to Symfony 2.8.3, same problem.