"Bosses require one or more closed contours that do not self-intersect"...problem??

Hi all,

So in the attached file, I'm getting the error message (in the title of my post) when I extrude "SKETCH Thick Line_Seed". This is confusing, as the sketch feature that I'm trying to extrude very much appears to be a closed contour.

I'm not entirely sure how to interpret a "self-intersecting" contour, so maybe that could be the problem...?

Any help is appreciated - thanks in advance !

SolidworksModeling And Assemblies