Cannot convert value null to text - pivot

I have a reasonably complex workflow with several tables and am getting the error below. Unfortunately,  I cannot share the workbook or data. 

I checked this thread but couldn't find a solution:  https://community.powerbi.com/t5/Power-Query/Expression-Error-We-cannot-convert-the-value-null-to-type-Number/td-p/1817857 

This error appeared after one change on "Metadata-Responses", where I replaced the 0 values will null (so it now has values 1-5 and null). This then caused the error two tables down the line, even thought the field that is altered in the first table is only used there and then deleted in the following table. The error appears on the very last step, which is a pivot  

Data that the pivot is performed on 

Pivot settings

•  The error does not appear when I delete the pivot step (but it causes a bunch of other errors because the data model doesn't work anymore) 
• The pivot worked fine before I added the one transformation step to the field two tables previous to this one and on a field that is no longer present in this table 
• I checked every step in these three tables to see if there is another error somewhere but there are none
• The altered field is not used in any other actions within Power Query