diff --git a/docs/PortKnocking.xml b/docs/PortKnocking.xml
index 8363f3674..e52c5c47b 100644
--- a/docs/PortKnocking.xml
+++ b/docs/PortKnocking.xml
@@ -108,7 +108,7 @@ if ( $level ) {
                     '',
                     $tag,
                     'add',
-                    '-p tcp --dport ! 22 ' );
+                    '-p tcp ! --dport 22 ' );
 }
 
 add_rule( $chainref, '-p tcp --dport 22   -m recent --rcheck --seconds 60 --name SSH          -j ACCEPT' );