Print while grouping and there's a float time

[nicothebrush]

Hello, I got an error if I group by my tree list and in the columns there's a float (but represented with widget float_time, also have sum parameter), in this case when I print (both PDF or XLS):

Error: '21:00' non è un float corretto (translation: 21:00 is not a correct float)
http://192.168.1.2:8069/web/webclient/js?db=Demo:3136

Thanks