diff --git a/src/_assert.ts b/src/_assert.ts
index 9826c22..fb75c97 100644
--- a/src/_assert.ts
+++ b/src/_assert.ts
@@ -1,13 +1,13 @@
 function number(n: number) {
-  if (!Number.isSafeInteger(n) || n < 0) throw new Error(`Wrong positive integer: ${n}`);
+  if (!Number.isSafeInteger(n) || n < 0) throw new Error(`positive integer expected, not ${n}`);
 }
 
 function bool(b: boolean) {
-  if (typeof b !== 'boolean') throw new Error(`Expected boolean, not ${b}`);
+  if (typeof b !== 'boolean') throw new Error(`boolean expected, not ${b}`);
 }
 
 // copied from utils
-function isBytes(a: unknown): a is Uint8Array {
+export function isBytes(a: unknown): a is Uint8Array {
   return (
     a instanceof Uint8Array ||
     (a != null && typeof a === 'object' && a.constructor.name === 'Uint8Array')
@@ -15,9 +15,9 @@ function isBytes(a: unknown): a is Uint8Array {
 }
 
 function bytes(b: Uint8Array | undefined, ...lengths: number[]) {
-  if (!isBytes(b)) throw new Error('Expected Uint8Array');
+  if (!isBytes(b)) throw new Error('Uint8Array expected');
   if (lengths.length > 0 && !lengths.includes(b.length))
-    throw new Error(`Expected Uint8Array of length ${lengths}, not of length=${b.length}`);
+    throw new Error(`Uint8Array expected of length ${lengths}, not of length=${b.length}`);
 }
 
 type Hash = {
diff --git a/src/utils.ts b/src/utils.ts
index ef9968e..03582db 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -7,6 +7,7 @@
 // Makes the utils un-importable in browsers without a bundler.
 // Once node.js 18 is deprecated (2025-04-30), we can just drop the import.
 import { crypto } from '@noble/hashes/crypto';
+import { isBytes, bytes as abytes } from './_assert.js';
 
 // prettier-ignore
 export type TypedArray = Int8Array | Uint8ClampedArray | Uint8Array |
@@ -17,13 +18,6 @@ export const u8 = (arr: TypedArray) => new Uint8Array(arr.buffer, arr.byteOffset
 export const u32 = (arr: TypedArray) =>
   new Uint32Array(arr.buffer, arr.byteOffset, Math.floor(arr.byteLength / 4));
 
-function isBytes(a: unknown): a is Uint8Array {
-  return (
-    a instanceof Uint8Array ||
-    (a != null && typeof a === 'object' && a.constructor.name === 'Uint8Array')
-  );
-}
-
 // Cast array to view
 export const createView = (arr: TypedArray) =>
   new DataView(arr.buffer, arr.byteOffset, arr.byteLength);
@@ -44,6 +38,8 @@ export const byteSwap = (word: number) =>
 // Conditionally byte swap if on a big-endian platform
 export const byteSwapIfBE = isLE ? (n: number) => n : (n: number) => byteSwap(n);
 
+export { isBytes };
+
 // In place byte swap for Uint32Array
 export function byteSwap32(arr: Uint32Array) {
   for (let i = 0; i < arr.length; i++) {
@@ -59,7 +55,7 @@ const hexes = /* @__PURE__ */ Array.from({ length: 256 }, (_, i) =>
  * @example bytesToHex(Uint8Array.from([0xca, 0xfe, 0x01, 0x23])) // 'cafe0123'
  */
 export function bytesToHex(bytes: Uint8Array): string {
-  if (!isBytes(bytes)) throw new Error('Uint8Array expected');
+  abytes(bytes);
   // pre-caching improves the speed 6x
   let hex = '';
   for (let i = 0; i < bytes.length; i++) {
@@ -136,7 +132,7 @@ export type Input = Uint8Array | string;
  */
 export function toBytes(data: Input): Uint8Array {
   if (typeof data === 'string') data = utf8ToBytes(data);
-  if (!isBytes(data)) throw new Error(`expected Uint8Array, got ${typeof data}`);
+  abytes(data);
   return data;
 }
 
@@ -147,7 +143,7 @@ export function concatBytes(...arrays: Uint8Array[]): Uint8Array {
   let sum = 0;
   for (let i = 0; i < arrays.length; i++) {
     const a = arrays[i];
-    if (!isBytes(a)) throw new Error('Uint8Array expected');
+    abytes(a);
     sum += a.length;
   }
   const res = new Uint8Array(sum);
diff --git a/test/utils.test.js b/test/utils.test.js
index 8f35634..5a8e9c9 100644
--- a/test/utils.test.js
+++ b/test/utils.test.js
@@ -1,7 +1,7 @@
 const assert = require('assert');
 const { should } = require('micro-should');
 const { optional, integer, gen } = require('./generator');
-const { byteSwap, byteSwapIfBE, byteSwap32, isLE } = require('../utils.js');
+const { byteSwap, byteSwapIfBE, byteSwap32, isLE } = require('../utils');
 
 // Here goes test for tests...
 should(`Test generator`, () => {