Invocation of a polymorphic field-like event

Considering the code below:

public class TableMain {
    public virtual event Action UpdateFilter;
    ....
}

public class TableSub : TableMain {
    public override event Action UpdateFilter;

    public void UpdateQuery() {
        .....
        if (UpdateFilter!=null) {
              UpdateFilter(); // Invocation of polymorphic field-like event???
        }
    }
}

In this code ReSharper shows the alert "invocation of polymorphic field-like event".

My question is: What does it actually mean? And is it an alert for a bad programming practice? Also, is it a bad practice to call an event polymorphically? (Knowing that an event can only be raised from the class which has declared it.)