From cce590e9f09a7e69a8b3eb073243ff5f7e81db29 Mon Sep 17 00:00:00 2001
From: "shuwen.wu" <shuwen.wu@bytedance.com>
Date: Fri, 25 Sep 2026 11:56:58 +0800
Subject: [PATCH] fix: handle None return from error handler in no-output
 background callbacks

When a background callback with no outputs raises an exception, the error
handler is called. If the error handler returns None, the code was
incorrectly setting output_value = NoUpdate(), which then triggered an
InvalidCallbackReturnValue exception at the output validation step.

This change adds the same output_spec check that exists in the non-background
callback path (line 866) to the background callback path (line 635), ensuring
that NoUpdate() is only set when there are actual outputs to update.

Fixes plotly/dash#3628
---
 dash/_callback.py | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/dash/_callback.py b/dash/_callback.py
index 1a24bdf621..c400a3c3a1 100644
--- a/dash/_callback.py
+++ b/dash/_callback.py
@@ -574,6 +574,7 @@ def _update_background_callback(
     kwargs,
     background,
     multi,
+    output_spec,
     cache_key=None,
     job_id=None,
 ):
@@ -602,6 +603,7 @@ def _update_background_callback(
         error_handler,
         callback_ctx,
         multi,
+        output_spec,
         cache_key=cache_key,
         job_id=job_id,
     )
@@ -614,6 +616,7 @@ def _handle_rest_background_callback(
     error_handler,
     callback_ctx,
     multi,
+    output_spec,
     has_update=False,
     cache_key=None,
     job_id=None,
@@ -632,7 +635,7 @@ def _handle_rest_background_callback(
         if error_handler:
             output_value = error_handler(exc)
 
-            if output_value is None:
+            if output_value is None and output_spec:
                 output_value = NoUpdate()
             # set_props from the error handler uses the original ctx
             # instead of manager.get_updated_props since it runs in the
@@ -852,7 +855,7 @@ def add_context(*args, **kwargs):
                         )
 
                     output_value, has_update, skip = _update_background_callback(
-                        error_handler, callback_ctx, response, kwargs, background, multi
+                        error_handler, callback_ctx, response, kwargs, background, multi, output_spec
                     )
                     if skip:
                         return output_value
@@ -923,7 +926,7 @@ async def async_add_context(*args, **kwargs):
                             callback_ctx,
                         )
                     output_value, has_update, skip = _update_background_callback(
-                        error_handler, callback_ctx, response, kwargs, background, multi
+                        error_handler, callback_ctx, response, kwargs, background, multi, output_spec
                     )
                     if skip:
                         return output_value