diff --git a/test/utils.js b/test/utils.js
index f8dd003..4efce10 100644
--- a/test/utils.js
+++ b/test/utils.js
@@ -21,3 +21,34 @@ export function json(path) {
     return require('./' + file + '.json'); // in this form so that bundler can glob this
   }
 }
+
+
+const TYPE_TEST_BASE = [
+  null,
+  [1, 2, 3],
+  { a: 1, b: 2, c: 3 },
+  NaN,
+  0.1234,
+  1.0000000000001,
+  10e9999,
+  new Uint32Array([1, 2, 3]),
+  100n,
+  new Set([1, 2, 3]),
+  new Uint8ClampedArray([1, 2, 3]),
+  new Int16Array([1, 2, 3]),
+  new ArrayBuffer(100),
+  new DataView(new ArrayBuffer(100)),
+  { constructor: { name: 'Uint8Array' }, length: '1e30' },
+  () => {},
+  async () => {},
+  class Test {},
+];
+const TYPE_TEST_NOT_STR = [
+  ' 1 2 3 4 5',
+  '010203040x',
+  'abcdefgh',
+  '1 2 3 4 5 ',
+  'bee',
+  new String('1234'),
+];
+export const TYPE_TEST = { hex: TYPE_TEST_BASE.concat(TYPE_TEST_NOT_STR), bytes: TYPE_TEST_BASE.concat(TYPE_TEST_NOT_STR) };
diff --git a/test/utils.test.js b/test/utils.test.js
new file mode 100644
index 0000000..659c7d9
--- /dev/null
+++ b/test/utils.test.js
@@ -0,0 +1,28 @@
+import { throws } from 'node:assert';
+import { describe, should } from 'micro-should';
+import { bytesToHex, hexToBytes } from '../esm/abstract/utils.js';
+import { TYPE_TEST } from './utils.js';
+
+// Here goes test for tests...
+describe('Tests', () => {
+  should('hexToBytes', () => {
+    for (let v of TYPE_TEST.hex) {
+      throws(() => {
+        hexToBytes(v);
+      });
+    }
+  });
+  should('bytesToHex', () => {
+    for (let v of TYPE_TEST.bytes) {
+      throws(() => {
+        bytesToHex(v);
+      });
+    }
+  });
+});
+
+// ESM is broken.
+import url from 'node:url';
+if (import.meta.url === url.pathToFileURL(process.argv[1]).href) {
+  should.run();
+}