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Hey all,

Can someone explain to me why I am getting this error:

```Traceback (most recent call last):
  File "runner.py", line 48, in inner
    new_state = method(self, state, *args, **kwargs)
  File "flow_runner.py", line 569, in get_flow_run_state
    executors.prepare_upstream_states_for_mapping(
  File "\executors.py", line 537, in prepare_upstream_states_for_
mapping
    value = upstream_state.result[i]
KeyError: 0
[2021-07-19 18:23:21+0200] ERROR - prefect.ETL | Unexpected error occured in FlowRunner: KeyError(0)```

Hey <@U01EBUU7Y8N>, it’s a bit hard to say. Is your Flow code simple enough to share?

I managed to find what was going wrong. I used a dictionary as input for mapping instead of a list.. :stuck_out_tongue: