diff --git a/pkgkernel.sh b/pkgkernel.sh
index 6fb1c06..8d17901 100755
--- a/pkgkernel.sh
+++ b/pkgkernel.sh
@@ -5,7 +5,8 @@ DEFAULT='\033[0m'
 PREFIX='linux-'
 EXTENSION='.tar.xz'
 # For RC releases
-if [ `expr index ${1} gz` -gt 0 ] ; then 
+
+if [ `echo ${1} | sed -n "s/gz.*//p" |wc -l` -gt 0 ] ; then 
     EXTENSION='.tar.gz'
 fi