[GIS] How to handle an “Input CRS error” when clipping

Question

I'm trying to clip a soil use shapefile, with a polygon I've created this morning. The clipping function works, but the clipped polygon doesn't have features (the table of contents is empty). QGIS advices me that:

*Created output shapefile:
/Users/biaginisandra/Desktop/Uso_suolo_Roncacce/Uso_suolo_Roncacce.shp
Warnings:
Input CRS error: One or more input layers missing coordinate reference information, results may not be as expected.
GEOS geoprocessing error: One or more input features have invalid geometry.
Some output geometries may be missing or invalid.
Would you like to add the new layer anyway?*

Where is the problem?

Thanks in advance.

Answer 1

"Defining" a CRS with Set CRS for layer is not enough for clipping, because clipping is an external command.

The shapefiles must have the same CRS. That can be applied with rightclick -> Save as... under a different name, but same CRS for both shapefiles.