diff --git a/mk_mmc.sh b/mk_mmc.sh
index 9bf4a2b..6b7053c 100755
--- a/mk_mmc.sh
+++ b/mk_mmc.sh
@@ -35,6 +35,7 @@ unset SMSC95XX_MOREMEM
 unset DD_UBOOT
 unset KERNEL_DEB
 unset USE_UENV
+unset ADDON
 
 GIT_VERSION=$(git rev-parse --short HEAD)
 IN_VALID_UBOOT=1
@@ -98,6 +99,22 @@ function is_element_of {
 	return 1
 }
 
+#########################################################################
+#
+#  Define valid "--addon" values.
+#
+#########################################################################
+
+VALID_ADDONS="pico ulcd"
+
+function is_valid_addon {
+	if is_element_of $1 "${VALID_ADDONS}" ] ; then
+		return 0
+	else
+		return 1
+	fi
+}
+
 function check_root {
 if [[ $UID -ne 0 ]]; then
  echo "$0 must be run as sudo user or root"
@@ -1659,29 +1676,6 @@ case "$UBOOT_TYPE" in
 	esac
 }
 
-function check_addon_type {
- IN_VALID_ADDON=1
-
-case "$ADDON_TYPE" in
-    pico)
-
- ADDON=pico
- unset IN_VALID_ADDON
-
-        ;;
-    ulcd)
-
- ADDON=ulcd
- unset IN_VALID_ADDON
-
-        ;;
-esac
-
- if [ "$IN_VALID_ADDON" ] ; then
-   usage
- fi
-}
-
 function check_distro {
  IN_VALID_DISTRO=1
 
@@ -1772,7 +1766,7 @@ Optional:
       precise-armel (12.04)
       precise-armhf (12.04)
 
---addon <device>
+--addon <additional peripheral device>
     pico
     ulcd <beagle xm>
 
@@ -1852,8 +1846,7 @@ while [ ! -z "$1" ]; do
             ;;
         --addon)
             checkparm $2
-            ADDON_TYPE="$2"
-            check_addon_type
+            ADDON=$2
             ;;
         --svideo-ntsc)
             SVIDEO_NTSC=1
@@ -1882,14 +1875,21 @@ while [ ! -z "$1" ]; do
     shift
 done
 
-if [ ! "${MMC}" ];then
-    echo "ERROR: --mmc undefined"
-    usage
+if [ ! "${MMC}" ] ; then
+	echo "ERROR: --mmc undefined"
+	usage
 fi
 
 if [ "$IN_VALID_UBOOT" ] ; then
-    echo "ERROR: --uboot undefined"
-    usage
+	echo "ERROR: --uboot undefined"
+	usage
+fi
+
+if [ -n ${ADDON} ] ; then
+	if ! is_valid_addon ${ADDON} ; then
+		echo "ERROR: ${ADDON} is not a valid addon type"
+		usage
+	fi
 fi
 
  echo ""