diff --git a/packages/ans/src/bytes.test.ts b/packages/ans/src/bytes.test.ts
new file mode 100644
index 0000000..ee1dfb0
--- /dev/null
+++ b/packages/ans/src/bytes.test.ts
@@ -0,0 +1,27 @@
+import { describe, expect, it } from 'vitest';
+import { fromHex, toHex } from './bytes.js';
+
+describe('fromHex', () => {
+  it('decodes valid hex, with and without 0x prefix', () => {
+    expect([...fromHex('00ff10')]).toEqual([0x00, 0xff, 0x10]);
+    expect([...fromHex('0xdeadbeef')]).toEqual([0xde, 0xad, 0xbe, 0xef]);
+    expect([...fromHex('')]).toEqual([]);
+  });
+
+  it('round-trips with toHex', () => {
+    const bytes = Uint8Array.from([0, 1, 127, 128, 255]);
+    expect([...fromHex(toHex(bytes))]).toEqual([...bytes]);
+  });
+
+  it('throws on odd-length input', () => {
+    expect(() => fromHex('abc')).toThrow(/odd-length/);
+  });
+
+  it('throws on non-hex characters instead of silently decoding to 0', () => {
+    // Previously parseInt('zz', 16) === NaN, which a Uint8Array stores as 0,
+    // so fromHex('zzzz') silently returned [0, 0].
+    expect(() => fromHex('zzzz')).toThrow(/invalid hex/);
+    expect(() => fromHex('00gg')).toThrow(/invalid hex/);
+    expect(() => fromHex('0xnothex')).toThrow(/invalid hex/);
+  });
+});
diff --git a/packages/ans/src/bytes.ts b/packages/ans/src/bytes.ts
index 8cc809f..058df41 100644
--- a/packages/ans/src/bytes.ts
+++ b/packages/ans/src/bytes.ts
@@ -26,6 +26,9 @@ export function toHex(bytes: Uint8Array): string {
 export function fromHex(hex: string): Uint8Array {
   const clean = hex.startsWith('0x') ? hex.slice(2) : hex;
   if (clean.length % 2 !== 0) throw new Error('odd-length hex string');
+  // Reject non-hex input. Without this, parseInt returns NaN for a bad pair
+  // and the Uint8Array silently stores 0, producing wrong bytes.
+  if (!/^[0-9a-fA-F]*$/.test(clean)) throw new Error('invalid hex string');
   const out = new Uint8Array(clean.length / 2);
   for (let i = 0; i < out.length; i++) out[i] = parseInt(clean.slice(i * 2, i * 2 + 2), 16);
   return out;