gdal - gdal2tiles.py generating black tiles from .img file

I have a source HFA Erdas Imagine (.img) file which I'm trying to convert into tiles using gdal2tiles.py. I'm running:

gdal2tiles.py -w google -g GOOGLEKEY input_file.img

The HTML is generated correctly, and the tiles are correctly positioned. However, I'm getting black tiles instead of the actual content of the .img file:

I've verified (using QGis and MapTiler) that the .img contains the correct data, i.e. the source file has raster data.

Any ideas? Should I transform the .img to something else? Am I missing an option for the command?