Improve last change

* lisp/calendar/time-date.el (date-days-in-month): Improve the
error message text and make sure MONTH is a number.  (Bug#40217)

2 files changed, 4 insertions, 3 deletions

diff --git a/lisp/calendar/time-date.el b/lisp/calendar/time-date.el
index 9b58a4884bc..eeb09926a6e 100644
--- a/lisp/calendar/time-date.el
+++ b/lisp/calendar/time-date.el
@@ -355,8 +355,8 @@ is output until the first non-zero unit is encountered."
 
 (defun date-days-in-month (year month)
   "The number of days in MONTH in YEAR."
-  (unless (<= 1 month 12)
-    (error "Month %s invalid" month))
+  (unless (and (numberp month) (<= 1 month 12))
+    (error "Month %s is invalid" month))
   (if (= month 2)
       (if (date-leap-year-p year)
           29
diff --git a/test/lisp/calendar/time-date-tests.el b/test/lisp/calendar/time-date-tests.el
index 9c90300cfe6..3eecc67eb53 100644
--- a/test/lisp/calendar/time-date-tests.el
+++ b/test/lisp/calendar/time-date-tests.el
@@ -32,7 +32,8 @@
   (should (= (date-days-in-month 2004 2) 29))
   (should (= (date-days-in-month 2004 3) 31))
   (should-not (= (date-days-in-month 1900 3) 28))
-  (should-error (date-days-in-month 2020 15)))
+  (should-error (date-days-in-month 2020 15))
+  (should-error (date-days-in-month 2020 'foo)))
 
 (ert-deftest test-ordinal ()
   (should (equal (date-ordinal-to-time 2008 271)