Problem with generate_xmp.py

I'm trying to do the following:

./generate_xml.py osm.xml my_osm.xml --dbname osm --user osm --password my_password --accept-none

And I get:

Traceback (most recent call last):
  File "./generate_xml.py", line 204, in <module>
    serialize(template_xml,options)
  File "./generate_xml.py", line 80, in serialize
    mapnik.load_map(m,xml,True)
RuntimeError: Failed to find font face '' in FontSet 'book-fonts'

What am I doing wrong?