Error while importing dxf to section wizard in Staad Pro from AutoCAD

I am analysing an RCC box girder for which some peculiar tapered sections have to be created. I created a tapered rectangular section in AutoCAD 2014. I used polyline to create the 2D and then extruded it to make it a solid 3D section. Now when I import this into section wizard, I get an error, "Import error". I tried saving the AutoCAD file in various formats of .dwg and .dxf but the same result. What is it that I am not doing right?