diff --git a/src/client/utils/computer.ts b/src/client/utils/computer.ts
index 26ffb7ee..33e4b433 100644
--- a/src/client/utils/computer.ts
+++ b/src/client/utils/computer.ts
@@ -11,7 +11,7 @@ export const OperationTypeUserDeposit = 3;
 export const userIdToBytes = (uid: string) => {
   // User IDs are uint64 decimal strings. Reject hex/exponent/whitespace that
   // BigNumber would otherwise coerce (e.g. '0x10' -> 16, '1e3' -> 1000).
-  if (typeof uid === 'string' && !/^\d+$/.test(uid)) {
+  if (typeof uid !== 'string' || !/^\d+$/.test(uid)) {
     throw new Error(`invalid user id: ${uid}`);
   }
   let x: BigNumber;
diff --git a/src/client/utils/encoder.ts b/src/client/utils/encoder.ts
index 00c1ddee..33361e9f 100644
--- a/src/client/utils/encoder.ts
+++ b/src/client/utils/encoder.ts
@@ -175,14 +175,13 @@ export class Encoder {
     o.keys.forEach(k => {
       // Buffer.from(k, 'hex') silently drops non-hex chars, so a typo'd key
       // would shift the whole framing and burn funds to a wrong key.
-      const kb = Buffer.from(k, 'hex');
-      if (kb.byteLength !== 32) throw new Error(`invalid output key ${k}`);
-      this.write(kb);
+      if (!/^[0-9a-fA-F]{64}$/.test(k)) throw new Error(`invalid output key ${k}`);
+      this.write(Buffer.from(k, 'hex'));
     });
 
     const maskHex = o.mask || '';
+    if (maskHex && !/^[0-9a-fA-F]{64}$/.test(maskHex)) throw new Error(`invalid output mask ${o.mask}`);
     const mask = maskHex ? Buffer.from(maskHex, 'hex') : Buffer.alloc(32, 0);
-    if (mask.byteLength !== 32) throw new Error(`invalid output mask ${o.mask}`);
     this.write(mask);
 
     const scriptHex = o.script || '';
diff --git a/test/mixin/codec.test.ts b/test/mixin/codec.test.ts
index 67b8b7b6..8a94517f 100644
--- a/test/mixin/codec.test.ts
+++ b/test/mixin/codec.test.ts
@@ -180,7 +180,7 @@ describe('transaction codec', () => {
     expect(() => encoder.encodeOutput({ type, amount: '1', keys: [] })).toThrow('invalid output type');
   });
 
-  it.each([['zz'], ['44'.repeat(31)], ['abc']])('rejects an output with a malformed key: %s', key => {
+  it.each(['zz', '44'.repeat(31), 'abc', `${'44'.repeat(32)}f`, `${'44'.repeat(32)}zz`])('rejects an output with a malformed key: %s', key => {
     const encoder = new Encoder(Buffer.alloc(0));
 
     // Buffer.from(key, 'hex') silently truncates, which would shift the
@@ -188,12 +188,21 @@ describe('transaction codec', () => {
     expect(() => encoder.encodeOutput({ amount: '1', keys: [key] })).toThrow('invalid output key');
   });
 
-  it.each(['zz', '44'.repeat(31)])('rejects an output with a malformed mask: %s', mask => {
+  it.each(['zz', '44'.repeat(31), `${'44'.repeat(32)}f`, `${'44'.repeat(32)}zz`])('rejects an output with a malformed mask: %s', mask => {
     const encoder = new Encoder(Buffer.alloc(0));
 
     expect(() => encoder.encodeOutput({ amount: '1', keys: [], mask })).toThrow('invalid output mask');
   });
 
+  it.each([undefined, '', 'Ab'.repeat(32)])('accepts uppercase hex and an optional mask: %s', mask => {
+    const encoder = new Encoder(Buffer.alloc(0));
+    encoder.encodeOutput({ amount: '1', keys: ['Ab'.repeat(32)], mask });
+    expect(new Decoder(encoder.buffer()).decodeOutput()).toMatchObject({
+      keys: ['ab'.repeat(32)],
+      mask: mask ? 'ab'.repeat(32) : '00'.repeat(32),
+    });
+  });
+
   it('round-trips sorted signature entries', () => {
     const signatures = { 2: '22'.repeat(64), 0: '11'.repeat(64) };
     const encoder = new Encoder(Buffer.alloc(0));
diff --git a/test/mixin/computer.test.ts b/test/mixin/computer.test.ts
index 1b8e416a..7aca4c1d 100644
--- a/test/mixin/computer.test.ts
+++ b/test/mixin/computer.test.ts
@@ -31,6 +31,10 @@ describe('computer utilities', () => {
     expect(checkSystemCallSize(Buffer.alloc(MAX_SOLANA_TX_SIZE + 1))).toBe(false);
   });
 
+  it.each([1, Number.MAX_SAFE_INTEGER + 1, BigInt(1), null, undefined, true])('rejects a non-string user ID: %s', userID => {
+    expect(() => userIdToBytes(userID as unknown as string)).toThrow('invalid user id');
+  });
+
   it('builds system call extras with and without a fee ID', () => {
     const base = Buffer.concat([userIdToBytes('1'), Buffer.from(parseUUID(callID)), Buffer.from([1])]);