diff --git a/src/DigiComp/ruleset.xml b/src/DigiComp/ruleset.xml
index 7c0515c..fa0b776 100644
--- a/src/DigiComp/ruleset.xml
+++ b/src/DigiComp/ruleset.xml
@@ -60,7 +60,7 @@
 
     <rule ref="Generic.PHP.ForbiddenFunctions">
         <properties>
-            <property name="forbiddenFunctions" type="array" value="sizeof=>count,delete=>unset,xdebug_break=>null,join=>implode"/>
+            <property name="forbiddenFunctions" type="array" value="sizeof=>count,delete=>unset,xdebug_break=>null,join=>implode,is_null=>null"/>
         </properties>
     </rule>
     <rule ref="Generic.Formatting.SpaceAfterCast">