Join attribute by location result lacks some features in QGIS 2.18

I'm new in QGIS and I'm trying to do a Join attribute by location and Count points in polygon using a polygon shapefile and a points shapefile. After running these tools, an error message is printed:

"QgsFeature.setGeometry(QgsGeometry): argument 1 has unexpected type 'NoneType' See log for more details"

Even tho it creates a shapefile, I look into the returning shapefile and it creates some features over the area where points are located but it doesn't create features where other points are located. It would be great to know the cause of this problem and how to fix it.

Answer

Could be an invalid geometry. Try running the check validity tool and fixing any errors that are shown