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*`DagsterStepOutputNotFoundError` for `op` which yields `AssetMaterialization`*

I have an `op` which doesn't have an output. I have added yielding of an `AssetMaterialization` and now I'm getting `DagsterStepOutputNotFoundError`. I've tried adding:
• `yield`
• `yield Output()`
• `yield Nothing`
I'm on `0.13.9`.

What does the `op` need yield in addition to the `AssertMaterialization`?

My solution to this was:
```yield Output(None)```
I understand that as soon as something is being explicitly yielded from an `op`, e.g. `AssetMaterialization`, the implicit conversion of the `op` return value (maybe `None` if the `op` returns nothing) to an `Output` can't happen. In this case it's required to explicitly yield some `Output`, even if it's `Output(None)`.

<https://docs.dagster.io/concepts/ops-jobs-graphs/op-events#outputs>

Yes, that’s exactly right.   The implicit return value of a function that does not return anything is `None`.  Also, as soon as your op yields an explicit event, it must also yield an explicit `Output` .  <https://docs.dagster.io/concepts/assets/asset-materializations#yielding-an-assetmaterialization-from-a-op>