diff --git a/src/lib/util/merge.js b/src/lib/util/merge.js
index d50329b56..3a6058ec7 100644
--- a/src/lib/util/merge.js
+++ b/src/lib/util/merge.js
@@ -1,11 +1,11 @@
 export default function merge(obj = { }, defaults) {
-  if (typeof obj !== 'object' || obj === null) {
-    obj = {};
-  }
+  // Copy `obj` instead of mutating it, so that a caller's options object is not
+  // modified as a side effect (and a frozen options object does not throw).
+  const result = (typeof obj !== 'object' || obj === null) ? {} : { ...obj };
   for (const key in defaults) {
-    if (typeof obj[key] === 'undefined') {
-      obj[key] = defaults[key];
+    if (typeof result[key] === 'undefined') {
+      result[key] = defaults[key];
     }
   }
-  return obj;
+  return result;
 }
diff --git a/test/validators.test.js b/test/validators.test.js
index 3d2c8e8b2..9bbbbd4ec 100644
--- a/test/validators.test.js
+++ b/test/validators.test.js
@@ -16218,4 +16218,17 @@ describe('Validators', () => {
       ],
     });
   });
+
+  it('should not mutate the options object passed to a validator', () => {
+    const options = {};
+    validator.isEmail('foo@bar.com', options);
+    assert.deepStrictEqual(options, {}, 'options object was mutated');
+  });
+
+  it('should accept a frozen options object', () => {
+    assert.strictEqual(
+      validator.isURL('https://example.com', Object.freeze({ require_tld: true })),
+      true
+    );
+  });
 });