diff --git a/src/lib/isDate.js b/src/lib/isDate.js
index 3a1e4afd2..25c48a216 100644
--- a/src/lib/isDate.js
+++ b/src/lib/isDate.js
@@ -31,6 +31,10 @@ export default function isDate(input, options) {
     if (options.strictMode && input.length !== options.format.length) return false;
     const formatDelimiter = options.delimiters
       .find(delimiter => options.format.indexOf(delimiter) !== -1);
+
+    // The format's delimiter is not one of the allowed delimiters, so no input can match it.
+    if (formatDelimiter === undefined) return false;
+
     const dateDelimiter = options.strictMode
       ? formatDelimiter
       : options.delimiters.find(delimiter => input.indexOf(delimiter) !== -1);
diff --git a/test/validators.test.js b/test/validators.test.js
index 98d2a12ff..ca2b49025 100644
--- a/test/validators.test.js
+++ b/test/validators.test.js
@@ -14671,6 +14671,31 @@ describe('Validators', () => {
         'abc.04.05.2024',
       ],
     });
+    test({
+      validator: 'isDate',
+      args: [{ format: 'DD.MM.YYYY' }], // '.' is not in the default delimiters
+      invalid: [
+        '29.02.2020',
+        '29/02/2020',
+        '29-02-2020',
+      ],
+    });
+    test({
+      validator: 'isDate',
+      args: ['YYYY.MM.DD'], // old format for backward compatibility
+      invalid: [
+        '2020.02.29',
+        '2020/02/29',
+      ],
+    });
+    test({
+      validator: 'isDate',
+      args: [{ format: 'YYYY-MM-DD', delimiters: ['/'] }],
+      invalid: [
+        '2020-02-29',
+        '2020/02/29',
+      ],
+    });
     // emulating Pacific time zone offset & time
     // which could potentially result in UTC conversion issues
     timezone_mock.register('US/Pacific');