diff --git a/packages/csv-stringify/lib/api/normalize_options.js b/packages/csv-stringify/lib/api/normalize_options.js
index 32e63033c..a1b58d70c 100644
--- a/packages/csv-stringify/lib/api/normalize_options.js
+++ b/packages/csv-stringify/lib/api/normalize_options.js
@@ -200,7 +200,7 @@ const normalize_options = function (opts) {
   if (options.cast === undefined || options.cast === null) {
     options.cast = {};
   } else {
-    // todo
+    options.cast = { ...options.cast };
   }
   // Normalize option cast.bigint
   if (options.cast.bigint === undefined || options.cast.bigint === null) {
diff --git a/packages/csv-stringify/test/option.cast.ts b/packages/csv-stringify/test/option.cast.ts
index 5d27be58a..158f99c8f 100644
--- a/packages/csv-stringify/test/option.cast.ts
+++ b/packages/csv-stringify/test/option.cast.ts
@@ -1,8 +1,19 @@
 import "should";
 import dedent from "dedent";
 import { stringify } from "../lib/index.js";
+import { stringify as stringifySync } from "../lib/sync.js";
 
 describe("Option `cast`", function () {
+  it("preserves frozen options in the sync API", function () {
+    const string = (value: string) => value.toUpperCase();
+    const cast = { string };
+    Object.freeze(cast);
+    for (let i = 0; i < 2; i++) {
+      stringifySync([["value", 42]], { cast }).should.equal("VALUE,42\n");
+    }
+    cast.should.eql({ string });
+  });
+
   describe("default", function () {
     it("default BigInt formatter", function (next) {
       stringify(