diff --git a/src/lib/isMACAddress.js b/src/lib/isMACAddress.js
index d87cd4aa7..7c348ee05 100644
--- a/src/lib/isMACAddress.js
+++ b/src/lib/isMACAddress.js
@@ -9,25 +9,23 @@ const macAddress64WithDots = /^([0-9a-fA-F]{4}\.){3}([0-9a-fA-F]{4})$/;
 
 export default function isMACAddress(str, options) {
   assertString(str);
-  if (options?.eui) {
-    options.eui = String(options.eui);
-  }
+  const eui = options?.eui ? String(options.eui) : undefined;
   /**
    * @deprecated `no_colons` TODO: remove it in the next major
   */
   if (options?.no_colons || options?.no_separators) {
-    if (options.eui === '48') {
+    if (eui === '48') {
       return macAddress48NoSeparators.test(str);
     }
-    if (options.eui === '64') {
+    if (eui === '64') {
       return macAddress64NoSeparators.test(str);
     }
     return macAddress48NoSeparators.test(str) || macAddress64NoSeparators.test(str);
   }
-  if (options?.eui === '48') {
+  if (eui === '48') {
     return macAddress48.test(str) || macAddress48WithDots.test(str);
   }
-  if (options?.eui === '64') {
+  if (eui === '64') {
     return macAddress64.test(str) || macAddress64WithDots.test(str);
   }
   return isMACAddress(str, { eui: '48' }) || isMACAddress(str, { eui: '64' });
diff --git a/test/validators.test.js b/test/validators.test.js
index 98d2a12ff..e326df3e5 100644
--- a/test/validators.test.js
+++ b/test/validators.test.js
@@ -1115,6 +1115,32 @@ describe('Validators', () => {
     });
   });
 
+  it('should validate MAC addresses with frozen EUI options', () => {
+    [48, '48', 64, '64'].forEach((eui) => {
+      const address = Number(eui) === 48 ? '01:23:45:67:89:ab' : '01:23:45:67:89:ab:cd:ef';
+      test({
+        validator: 'isMACAddress',
+        args: [Object.freeze({ eui })],
+        valid: [address, address.replace(/:/g, '-')],
+        invalid: ['invalid', Number(eui) === 48 ? '01:23:45:67:89:ab:cd:ef' : '01:23:45:67:89:ab'],
+      });
+      ['no_separators', 'no_colons'].forEach((option) => {
+        test({
+          validator: 'isMACAddress',
+          args: [Object.freeze({ eui, [option]: true })],
+          valid: [address.replace(/:/g, '')],
+          invalid: [address, 'invalid'],
+        });
+      });
+    });
+  });
+
+  it('should not modify MAC address options', () => {
+    const options = { eui: 48 };
+    validator.isMACAddress('01:23:45:67:89:ab', options);
+    assert.strictEqual(options.eui, 48);
+  });
+
   it('should validate MAC addresses without separator', () => {
     test({
       validator: 'isMACAddress',