diff --git a/src/lib/isISBN.js b/src/lib/isISBN.js
index 4499c59a0..882d86261 100644
--- a/src/lib/isISBN.js
+++ b/src/lib/isISBN.js
@@ -9,12 +9,13 @@ export default function isISBN(isbn, options) {
 
   // For backwards compatibility:
   // isISBN(str [, version]), i.e. `options` could be used as argument for the legacy `version`
-  const version = String(options?.version || options);
+  let version = typeof options === 'object' ? options?.version : options;
 
-  if (!(options?.version || options)) {
+  if (!version) {
     return isISBN(isbn, { version: 10 }) || isISBN(isbn, { version: 13 });
   }
 
+  version = String(version);
   const sanitizedIsbn = isbn.replace(/[\s-]+/g, '');
 
   let checksum = 0;
diff --git a/test/validators/isISBN.test.js b/test/validators/isISBN.test.js
index 99fb2e014..f4005ff6a 100644
--- a/test/validators/isISBN.test.js
+++ b/test/validators/isISBN.test.js
@@ -53,6 +53,30 @@ describe('isISBN', () => {
     });
   });
 
+  [
+    { description: 'null options', options: null },
+    { description: 'empty options', options: {} },
+    { description: 'an undefined version', options: { version: undefined } },
+    { description: 'a null version', options: { version: null } },
+  ].forEach(({ description, options }) => {
+    it(`should validate both ISBN versions with ${description}`, () => {
+      test({
+        validator: 'isISBN',
+        args: [options],
+        valid: [
+          '340101319X',
+          '9784873113685',
+        ],
+        invalid: [
+          '3423214121',
+          '9783836221190',
+          'foo',
+          '',
+        ],
+      });
+    });
+  });
+
   describe('(legacy syntax)', () => {
     it('should validate ISBNs', () => {
       test({