svn: Out of date: '/trunk' in transaction '2-1

When I commit the changes to the svn, I got:
svn ci .
Sending .
svn: Commit failed (details follow):
svn: Out of date: '/trunk' in transaction '2-1'
svn: Your commit message was left in a temporary file:
svn: '/Users/balaparanj/work/zepho/ard/svn-commit.tmp'

Out of date here means the repository version is not the same as the local version. It works after doing svn up.