unexpected curve bevel behavior

I want to model a chair by curve bevel, but the result is not expected. What I have messed up? Thanks.

chair.zip (72.5 KB)

Your curve is a 2D curve, change it to 3D
Then Apply its rotation (Ctrl+A)

Note: No need to supply a zip file, that is one more step someone has to take care of before they hace the file. Just save the blend with the ‘compress’ option enabled in the save options and post that.

if a BezierCircle is selected as the bevel object, even the 2d curve can get expected result, which is a puzzling demo. Maybe this has been mentioned in the wiki somewhere.