Cannot select polygon using QGIS

I've created a polygon from lines in a couple of cases where I'm unable to select the polygon using the standard Select Features button. I am able to select the feature using the Attributes Table, but in neither case am I able to actually cut and paste the polygon from that layer to another layer.

Any ideas?

(Using QGIS 2.18 on OSX)

Answer

Your polygon isn't closed. If you open the Log Messages Panel you'll see a message on the GEOS tab:

Exception: IllegalArgumentException: Points of LinearRing do not form a closed linestring