diff --git a/doc/gtkdoc.sh b/doc/gtkdoc.sh
index 86de5d4..34ecae5 100755
--- a/doc/gtkdoc.sh
+++ b/doc/gtkdoc.sh
@@ -83,7 +83,7 @@ while getopts "ciuP:" name; do
 			uninstall=1
 			;;
 		P)
-			PREFIX="$2"
+			PREFIX="$OPTARG"
 			;;
 		?)
 			_usage