Multiple IIF Statement

I need help with a multiple/nested IIF statement that I am using within a Form.

I have two combo boxs in which the user can select a date. I also have a text field in which is where I want to implement the IIF statement.

I need the textfield to first look at combo box A, in which if it does not equals = "01-Jan-1900", then take the value from combo box B. If both statements fail, then equals "No Working Date". Should be simple enough, but I can't get the below code to work:

 =IIf([ComboA].[Column](1)<>"01-Jan-1900",[ComboA].[Column](1), IIf([ComboB].[Column](1)<>"01/01/1900",[ComboB].[Column](1),"NO WORKING DATE"))

Any help would be much appreciated.