diff --git a/tornado/concurrent.py b/tornado/concurrent.py
index 2a0615621..e017ffd58 100644
--- a/tornado/concurrent.py
+++ b/tornado/concurrent.py
@@ -131,7 +131,7 @@ def wrapper(self: Any, *args: Any, **kwargs: Any) -> Future:
     if len(args) == 1:
         return run_on_executor_decorator(args[0])
     elif len(args) != 0:
-        raise ValueError("expected 1 argument, got %d", len(args))
+        raise ValueError("expected 1 argument, got %d" % len(args))
     return run_on_executor_decorator
 
 
diff --git a/tornado/test/concurrent_test.py b/tornado/test/concurrent_test.py
index f376f87d5..4a1dc5e73 100644
--- a/tornado/test/concurrent_test.py
+++ b/tornado/test/concurrent_test.py
@@ -227,5 +227,15 @@ async def f():
         self.assertEqual(result, 42)
 
 
+class RunOnExecutorArgValidationTest(AsyncTestCase):
+    def test_run_on_executor_bad_args(self):
+        with self.assertRaisesRegex(ValueError, "expected 1 argument, got 2"):
+            @run_on_executor(1, 2)
+            def f():
+                pass
+
+        pass
+
+
 if __name__ == "__main__":
     unittest.main()