diff --git a/src/utils.ts b/src/utils.ts
index a773431b..06185a6b 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -18,8 +18,12 @@ export function isJSONSerializable(value: any): boolean {
   if (value === undefined) {
     return false;
   }
+  // `null` is JSON serializable (`JSON.stringify(null) === "null"`).
+  if (value === null) {
+    return true;
+  }
   const t = typeof value;
-  if (t === "string" || t === "number" || t === "boolean" || t === null) {
+  if (t === "string" || t === "number" || t === "boolean") {
     return true;
   }
   if (t !== "object") {
diff --git a/test/utils.test.ts b/test/utils.test.ts
new file mode 100644
index 00000000..f3754934
--- /dev/null
+++ b/test/utils.test.ts
@@ -0,0 +1,42 @@
+import { describe, it, expect } from "vitest";
+import { isJSONSerializable } from "../src/utils.ts";
+
+describe("isJSONSerializable", () => {
+  it("treats primitives as serializable", () => {
+    expect(isJSONSerializable("foo")).toBe(true);
+    expect(isJSONSerializable(123)).toBe(true);
+    expect(isJSONSerializable(true)).toBe(true);
+  });
+
+  it("treats `null` as serializable without throwing", () => {
+    // Regression: previously this threw `Cannot read properties of null
+    // (reading 'buffer')` because `null` fell through to the `value.buffer`
+    // check. `JSON.stringify(null)` is valid (`"null"`).
+    // eslint-disable-next-line unicorn/no-null
+    expect(() => isJSONSerializable(null)).not.toThrow();
+    // eslint-disable-next-line unicorn/no-null
+    expect(isJSONSerializable(null)).toBe(true);
+  });
+
+  it("treats `undefined` as non-serializable", () => {
+    expect(isJSONSerializable(undefined)).toBe(false);
+  });
+
+  it("treats plain objects and arrays as serializable", () => {
+    expect(isJSONSerializable({ a: 1 })).toBe(true);
+    expect(isJSONSerializable([1, 2, 3])).toBe(true);
+  });
+
+  it("treats objects with a `toJSON` method as serializable", () => {
+    expect(isJSONSerializable(new Date())).toBe(true);
+  });
+
+  it("treats non-serializable values as non-serializable", () => {
+    expect(isJSONSerializable(() => {})).toBe(false);
+    expect(isJSONSerializable(Symbol("x"))).toBe(false);
+    expect(isJSONSerializable(10n)).toBe(false);
+    expect(isJSONSerializable(new Uint8Array([1, 2, 3]))).toBe(false);
+    expect(isJSONSerializable(new FormData())).toBe(false);
+    expect(isJSONSerializable(new URLSearchParams())).toBe(false);
+  });
+});