Opening with a non flat top cannot be mulled underneath another opening

[allopher]

Hello,

 

I'm trying to create a mulled unit (see image).  But am getting the error "Opening with a non flat top cannot be mulled underneath another opening".  The door and left window have already been mulled together.  As far as I can tell, their top is flat.  I get this error when trying to mull the window/door unit with the top window.  What is the correct way to do this?

[Gawdzira]

Visually they may look flat. Do they both have the same top height numbers in the DBX?

 

[allopher]

Thanks to you both.  Turns out the window / door combo were not the same height.  When i created a sample to demo the problem, this time with windows and doors with casings, it become much clearer that there was a height difference.  Checking the actual position would have been a simpler solution.  Newbie mistake.  By the way, what does DBX stand for?

[Ridge_Runner]

I had this happen recently on a similar situation. I kept thinking the dbx  warning related to a "curved top" problem, which I didn't have; wasn't that silly of me to think that?  That's what I get for thinking "inside the box."