svn diff after file svn copy returns an empty diff

If I copy a file from one directory to another where it does not exist, and then 
use svn diff on the file, then I get an empty diff. I would expect to get a diff with 
all the file contents. Here's a script that demonstrates the problem: 
 
#!/bin/sh 
 
REPOS=repo 
REPOS_URL=file://`pwd`/repo 
WC=wc 
 
echo "creating repository" 
svnadmin create ${REPOS} 
svn co ${REPOS_URL} ${WC} > /dev/null 
 
echo "creating the src_dir" 
svn mkdir ${WC}/src_dir > /dev/null 
echo foo > ${WC}/src_dir/foo 
svn add ${WC}/src_dir/foo > /dev/null 
svn ci -m "Creating the src_dir." ${WC} > /dev/null 
 
echo "creating the dest_dir" 
svn mkdir ${WC}/dest_dir > /dev/null 
svn ci -m "Creating the dest_dir" ${WC} > /dev/null 
svn up ${WC} > /dev/null 
 
echo "copying the file" 
svn copy ${WC}/src_dir/foo ${WC}/dest_dir 
 
echo "comparing the file" 
(cd ${WC}/dest_dir ; svn diff foo )