Filtered combo box list in userform?

Wonder if anyone can help with this.

I have Sheet1 with columns A, B and C containing data. Column D has data in some cells.

There is also a userform which has a combobox list which has a row source (column A) and three text boxes for the remaining cells that are Vlookups.

What I want to be able to do is filter that dropdown list so that it will only display rows where column D is blank.

Does that make sense?