diff --git a/doc/scripts/subst.sh b/doc/scripts/subst.sh
index c87596c..2d790a3 100755
--- a/doc/scripts/subst.sh
+++ b/doc/scripts/subst.sh
@@ -79,7 +79,7 @@ while getopts "ciuP:" name; do
 			uninstall=1
 			;;
 		P)
-			PREFIX="$2"
+			PREFIX="$OPTARG"
 			;;
 		?)
 			_usage