diff --git a/src/lint/engine/ArcanistLintEngine.php b/src/lint/engine/ArcanistLintEngine.php
--- a/src/lint/engine/ArcanistLintEngine.php
+++ b/src/lint/engine/ArcanistLintEngine.php
@@ -471,6 +471,8 @@
   }
 
   private function executeLinters(array $runnable) {
+    assert_instances_of($runnable, 'ArcanistLinter');
+
     $all_paths = $this->getPaths();
     $path_chunks = array_chunk($all_paths, 32, $preserve_keys = true);