Fail when converting Weka's ARFF (2.1) format to Mahout's Vector format.

[Sonia Ben Ticha]

Hello,
I have run the converter to convert a Weka ARFF file to a Mahout Vector , the 
weka file contains a missing(represent with '?' in weka) but the program failed 
and display the following error message:
   Exception in thread "main" java.lang.NumberFormatException : for input 
string : "?"

Questions?
1) Why the converter don't accept missing value ('?')? 
2) How can I convert a data file (movie lens) to vector to execute the fuzzy 
kmean algorithm? 

Thank you
Sonia