diff --git a/backup.sh b/backup.sh
index 88d7383..aa59a66 100755
--- a/backup.sh
+++ b/backup.sh
@@ -52,7 +52,7 @@ do
 
     #${sqldump_prefix} ${sql_item//:/ } | gzip > ${prefix}/${stamp}/${sql_item//:/.}.sql.gz
 
-    if [[ -f "${prefix}/incremental/${sql_item//:/.}.sql" ]]
+    if [[ -f "${prefix}/incremental/${sql_item//:/.}.sql.sign" ]]
     then
 	xsuffix=".diff"
 	${sqldump_prefix} ${sql_item//:/ } | rdiff delta ${prefix}/incremental/${sql_item//:/.}.sql.sign > ${prefix}/${stamp}/${sql_item//:/.}.sql.delta