Thanks so much for your pointer Marica - that violation is clear and obvious to me in retrospect, but a good relearning experience in any case
diff --git a/erpnext/stock/report/stock_ageing/stock_ageing.py b/erpnext/stock/report/stock_ageing/stock_ageing.py
index eea2312..723ed5c 100644
--- a/erpnext/stock/report/stock_ageing/stock_ageing.py
+++ b/erpnext/stock/report/stock_ageing/stock_ageing.py
@@ -187,7 +187,7 @@
transferred_item_details[(d.voucher_no, d.name)].append(fifo_queue.pop(0))
else:
# all from current batch
- flt(batch[0]) -= qty_to_pop
+ batch[0] -= qty_to_pop
transferred_item_details[(d.voucher_no, d.name)].append([qty_to_pop, batch[1]])
qty_to_pop = 0