diff --git a/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/task.yaml b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/task.yaml
index 2d8a303..71af44a 100644
--- a/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/task.yaml
+++ b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/task.yaml
@@ -14,6 +14,8 @@ plan: |
   3. Fix it at its source, in that shared function or the value it uses. Do
      not special-case currencies in `invoicing/invoice.py` or
      `invoicing/currency.py`: every caller of the shared function has the
-     same problem, and all of them must be fixed by your change.
+     same problem, and all of them must be fixed by your change. The rates in
+     `invoicing/currency.py` have up to six decimal places (JPY is 0.006712),
+     and no step may round a rate or fraction to fewer places than that.
   4. Run `python -m pytest -q` until every test passes.
   5. Call finish.