How to pass the current instance to FieldInfo.GetValue inside the instance?

I'm making an abstract class that I need to validate fields in it's descendants. All the validated fields are marked by an attribute and are suppose to be checked for nulls or empty. To do that I get all the fields in the class:

var fields = this.GetType().GetFields().Where(field => Attribute.IsDefined(field, typeof(MyAttribute))).ToList(); 

And then, for every FieldInfo I try to do this:

if (string.IsNullOrEmpty(field.GetValue(this).ToString()))
{
    // write down the name of the field
}

I get a System.NullReferenceException: object reference not set to an instance of an object.

I know I am suppose to pass the instance of a class to the GetValue method. But how can I pass the current instance (the one that's launching the logic)?

Or: Is there an other way of getting the field's value?