Formula field not resolving in reports

I am having problems getting a formula field to work in reports. The formula is to get the season of birth from a DOB field:

if((DOB.getMonth() < 3),"Winter",if((DOB.getMonth() < 6),"Spring",if((DOB.getMonth() < 9),"Summer",if((DOB.getMonth() < 12),"Autumn","Winter"))))

It works fine in the list, but when I try to report using a pie chart it reports "No data Available" and the legend has 1 item "Unknown".

Is this a known issue, or am I doing something wrong?