User API doesnt work if I am passing the username as parameter and it is numeric value

Hello All, 

User API point always gives me "user not found" if the username is just the numeric value. It works well if the username is alphanumeric - 2132132abc

curl -H "Rest-User-Token:**********" -H "Rest-Method: PUT" -d "" -X POST https://community.my.com/api.ashx/v2/users/2132132.xml

Any idea what could be causing the issue?

Any help would be greatly appreciated.

Thanks,

Kasi