diff --git a/install/lib/installer_base.lib.php b/install/lib/installer_base.lib.php
index 0021057cc55f50c162971e588f0b783b17f528fe..81b07cdc56643a8b317c081b808ae94d83115dcb 100644
--- a/install/lib/installer_base.lib.php
+++ b/install/lib/installer_base.lib.php
@@ -635,7 +635,7 @@ class installer_base {
 
 		//* If there are RBL's defined, format the list and add them to smtp_recipient_restrictions to prevent removeal after an update
 		$rbl_list = '';
-		if ($server_ini_array['mail']['realtime_blackhole_list'] != '') {
+		if (isset($server_ini_array['mail']['realtime_blackhole_list']) && $server_ini_array['mail']['realtime_blackhole_list'] != '') {
 			$rbl_hosts = explode(",",str_replace(" ", "", $server_ini_array['mail']['realtime_blackhole_list']));
 			foreach ($rbl_hosts as $key => $value) {
 				$rbl_list .= ", reject_rbl_client ". $value;