I am trying to use Flaten inside of Data Factory, I keep on getting the error
The inner value type of the 'Unroll by' structure '{Category Numbers}' must be array type (e.g. structue of arrays, array of arrays)
Whilst I just did a 'Split' operation, this is the preview of the column 'Cateogry Numbers' from the previous step.

Does anybody have any idea why this is happening?