Decimal.sqrt(0) failed when the context was not
explicitly supplied.
diff --git a/Misc/NEWS b/Misc/NEWS
index 3e95a44..39b32cf 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -27,6 +27,8 @@
 Library
 -------
 
+- Issue #2478: fix failure of decimal.Decimal(0).sqrt()
+
 - Issue #2432: give DictReader the dialect and line_num attributes
   advertised in the docs.