chkrootkit throws Signal 13 when searching through /var/tmp

chkrootkit throws Signal 13 when searching through /var/tmp

On my debian squeeze server in the chkrootkit log I get loads of these
errors:
/usr/bin/find: Prozeß "head" wurde durch das Signal 13 abgebrochen.
/usr/bin/find: Prozeß "head" wurde durch das Signal 13 abgebrochen.
which means
head terminated by signal 13
A google search gives a lot of same problems, but no solution.
It comes from these lines in /usr/sbin/chkrootkit:
if [ `echo abc | head -n 1` = "abc" ]; then
fileshead="`${find} ${ROOTDIR}tmp ${ROOTDIR}var/tmp ${findargs}
-type f -exec head -n 1 {} \; | $egrep '#!.*php' 2> /dev/null`"
else
fileshead="`${find} ${ROOTDIR}tmp ${ROOTDIR}var/tmp ${findargs}
-type f -exec head -1 {} \; | grep '#!.*php' 2> /dev/null`"
fi
when I enter directly as root:
/usr/bin/find /var/tmp -type f -exec head -1 {} \; | grep php 2>
/dev/null;date
I get the same errors. egrep instead makes no difference.