weird "must be an intance" error

considering this snippet:

valPrintlocation=int(request.POST.get('printlocation'))
if valPrintlocation==-1:
needsprinting=0
else:
needsprinting=1

Item = ItemTracking.objects.get(Barcode =
barcode)
Item.tid=valPrintlocation
Item.NeedsPrinting=needsprinting
...

can you see any reason i'm getting this error:

Cannot assign "-1": "ItemTracking.tid" must be a "printLocation"
instance.

I get the same error regardless of the value of valPrintlocation

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