diff --git a/src/cal-period.scm b/src/cal-period.scm
index 4b32b59..316b1f7 100644
--- a/src/cal-period.scm
+++ b/src/cal-period.scm
@@ -195,7 +195,7 @@
  (define (cal-month-in-period? p . ml)
    (let ((m (if (null? ml)
 		(*current-month*)
-		(car ml)))
+		(cal-ensure-month (car ml))))
 	 (before (cal-ensure-month (cal-period-before p) #t))
 	 (since (cal-ensure-month (cal-period-since p))))
      (and (or (not before)