Can't close a works order NULL value itemdbkey error

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Been puzzling over this for a couple of days now. 

We have a completed works order that cant be closed due to an error that comes up saying:

  • Cannot update Field (itemDBKey) with a Null value as it is marked as not accepting 'Null' values. 

I did get this on a different Works Order, but I managed to fix it due to the warehouse information not being there. But this WO seems to be different 

The only reference to itemDBKey is in the MFGRepTraceabilityEnquiry table, but that doesn't seem to have much listed (unless its linked to another table) 

Anyone experience this before and can explain what is causing this?