diff --git a/CHANGELOG.md b/CHANGELOG.md
index 705c8b4c10cd..276d55a8605c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@ Contributed:
 Changes:
 
 - Extract metadata v14 `BTreeSet` types into correct type
+- Assert to ensure that `[u8; <size>]` has exact sizes (non-u8a inputs)
 - Adjust `tx.paymentInfo` signing process
 - Add Kusama 9190 upgrade block
 - Add Westend 9200 upgrade block
diff --git a/packages/types-codec/src/extended/U8aFixed.spec.ts b/packages/types-codec/src/extended/U8aFixed.spec.ts
index 0aaa662b9608..b448af13ff11 100644
--- a/packages/types-codec/src/extended/U8aFixed.spec.ts
+++ b/packages/types-codec/src/extended/U8aFixed.spec.ts
@@ -35,13 +35,43 @@ describe('U8aFixed', (): void => {
         new Uint8Array([0x02, 0x03, 0x00, 0x00])
       );
     });
+
+    it('constructs when passed Uint8Array is >= length', (): void => {
+      expect(
+        new (U8aFixed.with(32))(registry, new Uint8Array([0x00, 0x01, 0x02, 0x03])).toU8a()
+      ).toEqual(
+        new Uint8Array([0x00, 0x01, 0x02, 0x03])
+      );
+      expect(
+        new (U8aFixed.with(32))(registry, new Uint8Array([0x00, 0x01, 0x02, 0x03, 0x04, 0x05])).toU8a()
+      ).toEqual(
+        new Uint8Array([0x00, 0x01, 0x02, 0x03])
+      );
+    });
+
+    it('constructs when passed string is === length', (): void => {
+      expect(
+        new (U8aFixed.with(32))(registry, '1234').toU8a()
+      ).toEqual(
+        new Uint8Array([49, 50, 51, 52])
+      );
+    });
+
+    it('fails construction when passed string is > length', (): void => {
+      expect(
+        () => new (U8aFixed.with(32))(registry, '0x000102030405').toU8a()
+      ).toThrow(/Expected input with 4 bytes/);
+      expect(
+        () => new (U8aFixed.with(256))(registry, '1363HWTPzDrzAQ6ChFiMU6mP4b6jmQid2ae55JQcKtZnpLGv')
+      ).toThrow(/Expected input with 32 bytes/);
+    });
   });
 
   describe('utils', (): void => {
     let u8a: U8aFixed;
 
     beforeEach((): void => {
-      u8a = new U8aFixed(registry, [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 32);
+      u8a = new U8aFixed(registry, [1, 2, 3, 4], 32);
     });
 
     it('limits the length', (): void => {
diff --git a/packages/types-codec/src/extended/U8aFixed.ts b/packages/types-codec/src/extended/U8aFixed.ts
index c592169e7caa..5248b004b27f 100644
--- a/packages/types-codec/src/extended/U8aFixed.ts
+++ b/packages/types-codec/src/extended/U8aFixed.ts
@@ -3,7 +3,7 @@
 
 import type { AnyU8a, CodecClass, Registry, U8aBitLength } from '../types';
 
-import { assert, u8aToU8a } from '@polkadot/util';
+import { assert, isU8a, u8aToU8a } from '@polkadot/util';
 
 import { Raw } from '../native/Raw';
 
@@ -16,7 +16,7 @@ function decodeU8aFixed (value: AnyU8a, bitLength: U8aBitLength): [AnyU8a, numbe
     return [new Uint8Array(byteLength), 0];
   }
 
-  assert(u8a.length >= byteLength, () => `Expected at least ${byteLength} bytes (${bitLength} bits), found ${u8a.length} bytes`);
+  assert(isU8a(value) ? u8a.length >= byteLength : u8a.length === byteLength, () => `Expected input with ${byteLength} bytes (${bitLength} bits), found ${u8a.length} bytes`);
 
   return [u8a.subarray(0, byteLength), byteLength];
 }