Check for a common user error with defaultdict().
diff --git a/Misc/NEWS b/Misc/NEWS
index 1f216f7..e628f44 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -103,6 +103,8 @@
 Extension Modules
 -----------------
 
+- collections.defaultdict() now verifies that the factory function is callable.
+
 - Bug #1486663: don't reject keyword arguments for subclasses of builtin
   types.