RAM Concept - Slab supported on walls - error message

Hello,

I'm new to RAM Concept.  I'm trying to model a simple slab supported on 12" thick walls.  As shown on the screenshot, I'm getting an error message stating "strip very narrow."  There must be something I'm doing wrong?  The error message comes after I attempt to generate the strips.  Design file is also attached.  The structure summary perspective layer seems to correctly show what I'm going for. 

Also, when should the line support be used?  If I'm using walls, I shouldn't need to use line support, correct?

slab3.cpt