Nullifying a metaverse atttibute in an Import FlowRule extension

Hellos.

What is the trick here? I have learned that we can fiddle a null value into a MV attribute value via a SYNC Rule by assigning a never populated MV (null) attribute to the target MV attribute. This works.

How do I do something similar in my FlowRule extension. I get exception errors saying null not allowed! On the MA Properties dialog I have no option about allowing nulls or not in this "Advanced" flow.

I am trying simple stuff like:

switch (FlowRuleName)
            {
                case "HaagaEmployeeID":
                    
                    //  the MetaVerse employeeID value
                    if (mventry["employeeID"].IsPresent)
                    {
                        // skip if this has been already added for Haaga ou
                        if (mventry["ou"].Value == "Haaga")
                        {
                            // done already, no change
                        }
                        else
                        {
                            // nullify. No values for other OUs!
                            mventry["employeeID"].Value = null;
                        }
                    }

                    else ....