diff --git a/lib/helpers/processConnectionOptions.js b/lib/helpers/processConnectionOptions.js
index a9d862b1030..1dbb767ebee 100644
--- a/lib/helpers/processConnectionOptions.js
+++ b/lib/helpers/processConnectionOptions.js
@@ -9,11 +9,12 @@ function processConnectionOptions(uri, options) {
     ? opts.readPreference
     : getUriReadPreference(uri);
 
+  const clonedOpts = clone(opts);
   const resolvedOpts = (readPreference && readPreference !== 'primary' && readPreference !== 'primaryPreferred')
-    ? resolveOptsConflicts(readPreference, opts)
-    : opts;
+    ? resolveOptsConflicts(readPreference, clonedOpts)
+    : clonedOpts;
 
-  return clone(resolvedOpts);
+  return resolvedOpts;
 }
 
 function resolveOptsConflicts(pref, opts) {
diff --git a/test/connection.test.js b/test/connection.test.js
index dcf4cf621c7..49f715a8f4a 100644
--- a/test/connection.test.js
+++ b/test/connection.test.js
@@ -1537,4 +1537,13 @@ describe('connections:', function() {
     });
     assert.deepEqual(m.connections.length, 0);
   });
+
+  describe('processConnectionOptions', function() {
+    it('should not throw an error when attempting to mutate unmutable options object gh-13335', async function() {
+      const m = new mongoose.Mongoose();
+      const opts = Object.preventExtensions({});
+      const conn = await m.connect('mongodb://localhost:27017/db?retryWrites=true&w=majority&readPreference=secondaryPreferred', opts);
+      assert.ok(conn);
+    });
+  });
 });