Convert to Sheet Metal, Unable to Thicken - How to Fix?

I'm having trouble with the sheet metal conversion of this part.  The parts boundaries are supposed to be defined by the two loft sketches.  When I do the sheet metal conversion, however, I get an "unable to thicken" error unless I activate the "Reverse thickness" option.  This forces the material outside the sketch, which is not what I want.  What's the solution?

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