diff --git a/packages/tlock/src/commitment.test.ts b/packages/tlock/src/commitment.test.ts
index fc11cb3..73d3b4c 100644
--- a/packages/tlock/src/commitment.test.ts
+++ b/packages/tlock/src/commitment.test.ts
@@ -8,6 +8,7 @@ import {
   encodeBidPreimage,
   fromHex,
   i128ToBeBytes,
+  isValidHex,
   toHex,
 } from "./commitment.js";
 
@@ -72,3 +73,14 @@ test("fromHex rejects odd-length and non-hex input", () => {
   assert.throws(() => fromHex("12 3"), /invalid hex characters/);
   assert.throws(() => fromHex("0x12gg"), /invalid hex characters/);
 });
+
+test("isValidHex agrees with fromHex's accept/reject decisions", () => {
+  for (const hex of ["abcdef", "ABCDEF", "0xAbCdEf", "0XABCDEF", "", "0x"]) {
+    assert.equal(isValidHex(hex), true, hex);
+    assert.doesNotThrow(() => fromHex(hex));
+  }
+  for (const hex of ["abc", "zz", "12 3", "0x12gg"]) {
+    assert.equal(isValidHex(hex), false, hex);
+    assert.throws(() => fromHex(hex));
+  }
+});
diff --git a/packages/tlock/src/commitment.ts b/packages/tlock/src/commitment.ts
index 9994867..bc1414c 100644
--- a/packages/tlock/src/commitment.ts
+++ b/packages/tlock/src/commitment.ts
@@ -77,6 +77,13 @@ export function toHex(bytes: Uint8Array): string {
     .join("");
 }
 
+/// True iff `hex` is a valid hex string (optionally 0x-prefixed) that fromHex
+/// would accept: even length, and every remaining character is a hex digit.
+export function isValidHex(hex: string): boolean {
+  const clean = /^0x/i.test(hex) ? hex.slice(2) : hex;
+  return clean.length % 2 === 0 && /^[0-9a-fA-F]*$/.test(clean);
+}
+
 export function fromHex(hex: string): Uint8Array {
   const clean = /^0x/i.test(hex) ? hex.slice(2) : hex;
   if (clean.length % 2 !== 0) throw new Error("odd hex length");
diff --git a/packages/tlock/src/index.ts b/packages/tlock/src/index.ts
index 0cdd65f..36d1a49 100644
--- a/packages/tlock/src/index.ts
+++ b/packages/tlock/src/index.ts
@@ -6,6 +6,7 @@ export {
   beBytesToI128,
   toHex,
   fromHex,
+  isValidHex,
   VALUE_BYTES,
   NONCE_BYTES,
   PREIMAGE_BYTES,
diff --git a/packages/tlock/src/seal.test.ts b/packages/tlock/src/seal.test.ts
index 66543ca..224c302 100644
--- a/packages/tlock/src/seal.test.ts
+++ b/packages/tlock/src/seal.test.ts
@@ -99,6 +99,24 @@ test(
   },
 );
 
+test("sealBid rejects a non-positive Drand round", async () => {
+  const client = quicknet();
+  for (const round of [0, -1, -100]) {
+    await assert.rejects(
+      sealBid({ value: 1n, nonce: generateNonce(), round, client }),
+      /round must be a positive integer/,
+    );
+  }
+});
+
+test("openBid rejects an empty ciphertext", async () => {
+  const client = quicknet();
+  await assert.rejects(
+    openBid(new Uint8Array(0), client),
+    /ciphertext must not be empty/,
+  );
+});
+
 test(
   "a bid sealed to a future round cannot be opened — the seal holds",
   { timeout: NET_TIMEOUT },
diff --git a/packages/tlock/src/seal.ts b/packages/tlock/src/seal.ts
index 0cfdf83..b118434 100644
--- a/packages/tlock/src/seal.ts
+++ b/packages/tlock/src/seal.ts
@@ -42,6 +42,10 @@ export function generateNonce(): Uint8Array {
 export async function sealBid(params: SealBidParams): Promise<SealedBid> {
   const { value, nonce, round, client, identity, auditorPublicKey } = params;
 
+  if (!Number.isInteger(round) || round <= 0) {
+    throw new Error(`round must be a positive integer, got ${round}`);
+  }
+
   const preimage = encodeBidPreimage(value, nonce);
   const h = commitment(value, nonce);
   const armored = await timelockEncrypt(round, TlockBuffer.from(preimage), client);
@@ -68,6 +72,10 @@ export async function openBid(
   ciphertext: Uint8Array,
   client: DrandClient,
 ): Promise<OpenedBid> {
+  if (ciphertext.length === 0) {
+    throw new Error("ciphertext must not be empty");
+  }
+
   const armored = utf8Decode.decode(ciphertext);
   const plaintext = await timelockDecrypt(armored, client);
   return decodeBidPreimage(Uint8Array.from(plaintext));