diff --git a/lib/os/class.ISPConfigBaseOS.inc.php b/lib/os/class.ISPConfigBaseOS.inc.php
index 8d06c441f340c17da21a72228406dd446a69a7fd..919fce71defd872713cd0aa161684936c56fae67 100644
--- a/lib/os/class.ISPConfigBaseOS.inc.php
+++ b/lib/os/class.ISPConfigBaseOS.inc.php
@@ -127,7 +127,7 @@ class ISPConfigBaseOS {
 		}
 
 		foreach($entries as $line) {
-			if($add_if_existing === true || !preg_match('/^' . preg_quote($line) . '$/m', $content)) {
+			if($add_if_existing === true || !preg_match('/^' . preg_quote($line, '/') . '$/m', $content)) {
 				$content .= "\n" . $line . "\n";
 			}
 		}