diff --git a/Lib/calendar.py b/Lib/calendar.py
index ab14c7d..2e45e24 100644
--- a/Lib/calendar.py
+++ b/Lib/calendar.py
@@ -564,6 +564,10 @@
 firstweekday = c.getfirstweekday
 
 def setfirstweekday(firstweekday):
+    try:
+        firstweekday.__index__
+    except AttributeError:
+        raise IllegalWeekdayError(firstweekday)
     if not MONDAY <= firstweekday <= SUNDAY:
         raise IllegalWeekdayError(firstweekday)
     c.firstweekday = firstweekday