diff --git a/zfs-backup.sh b/zfs-backup.sh
index d79a07b..bda9a1e 100644
--- a/zfs-backup.sh
+++ b/zfs-backup.sh
@@ -143,7 +143,7 @@ function backup() {
 
 			if [ $_DELTA -lt 0 ]
 				then
-					_DELTA=$(( _$DELTA * -1 ))
+					_DELTA=$(( $_DELTA * -1 ))
 					WHAT="freed"
 			fi