Hi @lukezw,
The error is happening because the rasters downloaded directly from EarthStat do not have a nodata value set. The model shouldn’t care about that, it’s a small bug that I have targeted to fix in the next release. But in the meantime, you can get around it by setting a nodata value for the raster. Here are instructions for how to do that in QGIS. I recommend using a nodata value of -1 so it won’t conflict with the real data.
Hope that helps, and thanks for reporting these issues!