diff --git a/lib/utils/ascii85.test.ts b/lib/utils/ascii85.test.ts
new file mode 100644
index 0000000..b643c87
--- /dev/null
+++ b/lib/utils/ascii85.test.ts
@@ -0,0 +1,16 @@
+import { ascii85Decode } from './ascii85';
+
+describe('ascii85Decode', () => {
+  it('decodes a full 5-character group', () => {
+    expect(ascii85Decode('<~9jqo^~>')).toEqual(
+      new Uint8Array([0x4d, 0x61, 0x6e, 0x20]),
+    );
+  });
+
+  it.each(['<~uu~>', '<~uuu~>', '<~uuuu~>'])(
+    'rejects the short tail group %s whose padded value overflows uint32',
+    (input) => {
+      expect(ascii85Decode(input)).toBeNull();
+    },
+  );
+});
diff --git a/lib/utils/ascii85.ts b/lib/utils/ascii85.ts
index 5a3a7fe..0843ba8 100644
--- a/lib/utils/ascii85.ts
+++ b/lib/utils/ascii85.ts
@@ -34,10 +34,10 @@ export function ascii85Decode(input: string): Uint8Array | null {
       value = value * 85 + digit;
     }
 
-    if (chunkLen === 5 && value > 0xffffffff) return null;
+    // Applies to padded short tails too: a group whose value exceeds uint32 is invalid input.
+    if (value > 0xffffffff) return null;
 
-    // Use >>> 0 to simulate uint32 overflow for padded groups
-    const v = value >>> 0;
+    const v = value;
     const numBytes = chunkLen === 5 ? 4 : chunkLen - 1;
     if (numBytes >= 1) output.push((v >>> 24) & 0xff);
     if (numBytes >= 2) output.push((v >>> 16) & 0xff);