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
@@ -376,6 +376,9 @@
       'disabled' => ArcanistLintSeverity::SEVERITY_DISABLED,
     );
 
+    if (empty($match['severity'])) {
+      return ArcanistLintSeverity::SEVERITY_ERROR;
+    }
     $severity_name = strtolower(idx($match, 'severity'));
 
     foreach ($map as $name => $severity) {