I used the sample files located at this URL to learn QGIS: https://qgis.org/downloads/data/.
I tried to clip SR_50M_alaska_nad.tif using a shapefile as a mask (alaska.shp). The problem is I got this error which states that:
Ring Self-intersection at or near point .... Error 1: Cutline polygon is invalid.
How do I solve my problem using QGIS 2.18.9?