Room is too complex to construct

I am having difficulties auto defining rooms the design database.

After following the tutorial video, I have set up the layers on the dxf as walls, partitions, doors and glazing as instructed. The layers are highlighted as you would expect them to be. However when I attempt to autodefine a room, all I get is the above message.

I understand that the most common reason for seeing this error message is that there is a gap somewhen in the space you are trying to autodefine, so I selected a room  that was partition walls only and even removed the door for good measure, then polylined the inside wall line, tracing the existing dxf. After checking it was sealed, I ran the autodefine again and still got the same message. 

Are there any other reasons why this message should be displayed? I am struggling to understand why a room made of only 4 walls could be too complex to define?

Thanks in advance