diff --git a/lisp/calendar/calendar.el b/lisp/calendar/calendar.el
index 345687d1775..7c883617aca 100644
--- a/lisp/calendar/calendar.el
+++ b/lisp/calendar/calendar.el
@@ -2459,19 +2459,22 @@ Returns the corresponding Gregorian date."
 
 (defun calendar-date-is-valid-p (date)
   "Return t if DATE is a valid date."
-  (let ((month (calendar-extract-month date))
-        (day (calendar-extract-day date))
-        (year (calendar-extract-year date)))
-    (and (<= 1 month) (<= month 12)
-         ;; (calendar-read-date t) used to return a date with day = nil.
-         ;; Should not be valid (?), since many funcs prob assume integer.
-         ;; (calendar-read-date 'noday) returns (month year), which
-         ;; currently results in calendar-extract-year returning nil.
-         day year (<= 1 day) (<= day (calendar-last-day-of-month month year))
-         ;; BC dates left as non-valid, to suppress errors from
-         ;; complex holiday algorithms not suitable for years BC.
-         ;; Note there are side effects on calendar navigation.
-         (<= 1 year))))
+  (when (and (listp date)
+             (length= date 3))
+    (let ((month (calendar-extract-month date))
+          (day (calendar-extract-day date))
+          (year (calendar-extract-year date)))
+      (and (integerp month) (integerp day) (integerp year)
+           (<= 1 month) (<= month 12)
+           ;; (calendar-read-date t) used to return a date with day = nil.
+           ;; Should not be valid (?), since many funcs prob assume integer.
+           ;; (calendar-read-date 'noday) returns (month year), which
+           ;; currently results in calendar-extract-year returning nil.
+           day year (<= 1 day) (<= day (calendar-last-day-of-month month year))
+           ;; BC dates left as non-valid, to suppress errors from
+           ;; complex holiday algorithms not suitable for years BC.
+           ;; Note there are side effects on calendar navigation.
+           (<= 1 year)))))
 
 (defun calendar-date-equal (date1 date2)
   "Return t if the DATE1 and DATE2 are the same."
diff --git a/test/lisp/calendar/calendar-tests.el b/test/lisp/calendar/calendar-tests.el
new file mode 100644
index 00000000000..c41f14d3b54
--- /dev/null
+++ b/test/lisp/calendar/calendar-tests.el
@@ -0,0 +1,34 @@
+;;; calendar-tests.el --- tests for calendar/calendar.el  -*- lexical-binding:t -*-
+
+;; Copyright (C) 2024 Free Software Foundation, Inc.
+
+;; Author: Richard Lawrence <rwl@recursewithless.net>
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Code:
+
+(require 'ert)
+(require 'calendar)
+
+(ert-deftest calendar-test-validity-predicate ()
+  (should (eq (calendar-date-is-valid-p nil) nil))
+  (should (eq (calendar-date-is-valid-p "invalid") nil))
+  (should (eq (calendar-date-is-valid-p (list 1 2)) nil))
+  (should (eq (calendar-date-is-valid-p (list 5 1 2025)) t)))
+
+(provide 'calendar-tests)
+;;; calendar-tests.el ends here