Laugh And Point At The Syntax Moron

Feel free to pick on me. I can't get syntax right. Ever. So when I can't figure it out I just try to find an example of what I want to do and use it as a guide. But I've not been able to find a single example (and I looked at a lot of threads and the *shudder* help files) of using "OR" in a DSum within a single field. Here is a simplified version of my statement:

=DSum("[Contribution_Amount]","tbl_CONTRIBUTION","[Employee_ID] =" & Forms!frmNew_Contribution!Employee_ID & " and [Special_Case] Is Null")

This works perfectly. I want it to look like this:

=DSum("[Contribution_Amount]","tbl_CONTRIBUTION","[Employee_ID] =" & Forms!frmNew_Contribution!Employee_ID & " and [Special_Case] = 'stringABC' or Is Null")

But this errors. And I've tried every combination of ' " () & # I can think of and everything errors, unless I completely set the OR apart into a new criteria, in which case it executes but produces results that are not what I'm trying to get. It also works if I add "= 'stringABC'" and remove the IsNull. I just can't seem to have both. Does anyone know how to write this properly?