This is an easily fixable bug and I urge you file a JIRA (note the new Apache JIRA link). Till that fix is completed, the only workaround for you is to either not use those columns or ensure each group has at least 2 values for a categorical variable. 
 
Tetsuo, thx for finding and reporting a bug!
Rahul, thx for triaging! 

Cheers,
Ivan