为什么这个匀称的多边形无效？

Question

from shapely.wkt import loads
polygon = loads("POLYGON Z ((11.2704142515539 8.935877943566396 -16.74699224693797, 9.552838743503662 9.966533722939751 -16.77349242463657, 7.038910673892143 11.41232301137523 -16.7241410803892, 5.216749880203796 12.46026879775946 -16.68836982474948, 8.001220401932684 17.16509576267924 -16.77314058305908, 13.96493422834735 13.65121729926299 -16.69268953378357, 11.2704142515539 8.935877943566396 -16.74699224693797), (9.548433717869264 9.955660336966762 -16.73313716982752, 12.17899487645576 14.57302531902789 -16.73313714768369, 9.676190207235408 15.98563829772018 -16.7331371381166, 7.056180834251874 11.41237681923199 -16.733137145912, 9.64504281597692 15.6700635337078 -16.73313699214635, 11.91573358793374 14.30274366995717 -16.73313697544633, 9.548433717869264 9.955660336966762 -16.73313716982752))")
print(polygon.is_valid)