What it means
Somewhere in the graph, code tried to use a value that turned out to be None (nothing). In ComfyUI this typically means an upstream node did not produce the output a downstream node needed — it failed quietly, was skipped, or an optional input the node actually relies on was left unconnected.
The browser message is generic, so the useful detail is in the server console: the traceback names the node and attribute, which points to which connection produced None.
Common causes
- An upstream node returned nothing (failed or produced an empty result).
- An input the node needs was left unconnected.
- A model, conditioning, or image was not actually loaded before use.
- A bug in a custom node that returns None on an edge case.
How to fix it
- 1
Read the traceback in the server console
The console traceback names the node and the attribute accessed on None — that identifies which upstream connection produced nothing.
- 2
Confirm the upstream node produced output
Check that the node feeding the failing input actually ran and returned a value; wire in a valid source if it did not.
- 3
Connect the expected input
Provide the input the node relies on (even one marked optional may be required for the path you are using).
- 4
Update the custom node if it is the source
If the traceback points inside a custom node, update it — None-return edge cases are common upstream bug fixes.
Frequently asked questions
Frequently Asked Questions
Quick answers to the most common questions.
Related errors
Make every ComfyUI run reproducible
Numonic captures the full workflow behind each ComfyUI generation — models, samplers, seeds, prompts, and custom nodes — so a setup that finally works is one you can find and rebuild, not re-debug.