Shell closedness

I have created a Shell by sewing faces together. Once all the faces sewed in it should be a closed shell.
Using ShapeAnalysis_Shell I check for bad edges and free edges. There is none.

However, when I check shell.Closed() I get 0.How can that happen when there are no free edges?

I can make a solid out of this, export it to proE and view that as a solid object, which confirms that it is closed.