diff --git a/src/lint/linter/ArcanistScriptAndRegexLinter.php b/src/lint/linter/ArcanistScriptAndRegexLinter.php
--- a/src/lint/linter/ArcanistScriptAndRegexLinter.php
+++ b/src/lint/linter/ArcanistScriptAndRegexLinter.php
@@ -376,7 +376,7 @@
       'disabled' => ArcanistLintSeverity::SEVERITY_DISABLED,
     );
 
-    $severity_name = strtolower(idx($match, 'severity'));
+    $severity_name = strtolower(idx($match, 'severity', ''));
 
     foreach ($map as $name => $severity) {
       if (!empty($match[$name])) {