diff --git a/README.md b/README.md
index 4437a31..37cadd8 100644
--- a/README.md
+++ b/README.md
@@ -69,7 +69,7 @@ Most variables are the same as in the [official postgres image](https://hub.dock
 | POSTGRES_DB | Comma or space separated list of postgres databases to backup. Required. |
 | POSTGRES_DB_FILE | Alternative to POSTGRES_DB, but with one database per line, for usage with docker secrets. |
 | POSTGRES_EXTRA_OPTS | Additional options for `pg_dump`. Defaults to `-Z9`. |
-| POSTGRES_CLUSTER | Set to `TRUE` in order to use `pg_dumpall` instead. |
+| POSTGRES_CLUSTER | Set to `TRUE` in order to use `pg_dumpall` instead. Also set POSTGRES_EXTRA_OPTS to any value or empty since the default value is not compatible with `pg_dumpall`. |
 | POSTGRES_HOST | Postgres connection parameter; postgres host to connect to. Required. |
 | POSTGRES_PASSWORD | Postgres connection parameter; postgres password to connect with. Required. |
 | POSTGRES_PASSWORD_FILE | Alternative to POSTGRES_PASSWORD, for usage with docker secrets. |
diff --git a/backup.sh b/backup.sh
index 40dcc33..4811b78 100755
--- a/backup.sh
+++ b/backup.sh
@@ -72,7 +72,7 @@ for DB in ${POSTGRES_DBS}; do
   #Create dump
   if [ "${POSTGRES_CLUSTER}" = "TRUE" ]; then
     echo "Creating cluster dump of ${DB} database from ${POSTGRES_HOST}..."
-    pg_dumpall -l "${DB}" -f "${DFILE}" ${POSTGRES_EXTRA_OPTS}
+    pg_dumpall -l "${DB}" ${POSTGRES_EXTRA_OPTS} | gzip -9 > "${DFILE}"
   else
     echo "Creating dump of ${DB} database from ${POSTGRES_HOST}..."
     pg_dump -d "${DB}" -f "${DFILE}" ${POSTGRES_EXTRA_OPTS}