[BUG] Exceptions in Background Callback Cause InvalidCallbackReturnValue When Using Custom Error Handler #3628

Environment

• The server runs on Debian 13 on x86.
• Python 3.14.3 (main, Feb 3 2026, 22:52:18) [Clang 21.1.4 ] on linux
• The issue is not frontend-related.

Packages

dash                 4.0.0

Bug Description

When using a custom error handler with a background callback with no outputs, exceptions raised in that callback cause an InvalidCallbackReturnValue exception to be raised.

Expected Behavior

Exceptions raised in the background callback are handled by the custom error handler. No further exceptions occur.

Actual Behavior

After the exception is handled by the custom error handler as expected, Dash’s error handling code replaces the output value with NoUpdate(). Later on the output value is expected to be None because the callback has no outputs, so an InvalidCallbackReturnValue exception is raised.

Minimal Working Example

1. Install Dash and diskcache:

   pip install dash[diskcache]

2. Create a file main.py with the following contents:

   import diskcache
   from dash import Dash, DiskcacheManager, Input, callback, dcc, html
   
   cache = diskcache.Cache("./cache")
   background_callback_manager = DiskcacheManager(cache)
   
   
   def custom_error_handler(err):
       print("The app raised an exception.")
   
   
   app = Dash(
       background_callback_manager=background_callback_manager,
       on_error=custom_error_handler,
   )
   
   app.layout = html.Div(
       [
           dcc.Input(id="input-number", type="number", value=1),
       ]
   )
   
   
   @callback(
       Input("input-number", "value"),
       background=True,
   )
   def update_output(_value):
       raise ValueError(
           "This exception is supposed to be caught by custom_error_handler and not shown "
           "in the developer tools."
       )
   
   
   if __name__ == "__main__":
       app.run(debug=True)

3. Run the application:

   python3.14 main.py

4. Open the web application in your browser:

   firefox http://127.0.0.1:8050/

5. Cause the exception by clicking a spinner button.

Analysis

The issue does not arise if background is set to False because the output of the error handler is treated differently in each case. In both cases, the error handler is called on the error and its return value is written to output_value.

• When background=False, output_value is left as is here because output_spec is Falsy:

  if output_value is None and output_spec:
      output_value = NoUpdate()

• When background=True, output_value is replaced here because output_spec is not considered:

  if output_value is None:
      output_value = NoUpdate()

I do not know the rationale behind this decision, but it appears to break the intended behavior.