diff --git a/scripts/cls2tex.sh b/scripts/cls2tex.sh
index a085ba5..3bcd9ce 100755
--- a/scripts/cls2tex.sh
+++ b/scripts/cls2tex.sh
@@ -60,10 +60,9 @@ while true ; do
 			exit
 			;;
 		-p|--prefix)
+			PREFIX=$2
 			shift
-			PREFIX=$1
 			shift
-			continue
 			;;
 
 		# handle unknown options...
diff --git a/scripts/make-images.sh b/scripts/make-images.sh
index 9a352c6..7be3a53 100644
--- a/scripts/make-images.sh
+++ b/scripts/make-images.sh
@@ -177,6 +177,12 @@ while true ; do
 			shift
 			;;
 
+		# handle unknown options...
+		-*|--*)
+			echo "Error: unknown option \"$1\""
+			exit
+			;;
+
 		*)
 			break
 			;;