diff --git a/mk_mmc.sh b/mk_mmc.sh
index 31f1406..eaf22d2 100755
--- a/mk_mmc.sh
+++ b/mk_mmc.sh
@@ -1872,7 +1872,7 @@ if [ ! "${MMC}" ] ; then
 	usage
 fi
 
-if [ "$IN_VALID_UBOOT" ] ; then
+if [ "${IN_VALID_UBOOT}" ] ; then
 	echo "ERROR: --uboot undefined"
 	usage
 fi