diff --git a/CHANGELOG.md b/CHANGELOG.md
index d53cecd..3f80e77 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,7 +2,7 @@
 
 ### Fix
 
-- set environment in create_release_pr (#46)
+- fix encoder buffer overflow when a map value fills the initial 2 KB buffer exactly (#59)
 
 ### Refactor
 
diff --git a/src/encode/encode.spec.ts b/src/encode/encode.spec.ts
index 2381128..b3be18b 100644
--- a/src/encode/encode.spec.ts
+++ b/src/encode/encode.spec.ts
@@ -1,5 +1,6 @@
-import { it, describe, expect } from 'vitest';
+import { it, describe, expect, test, vi, beforeEach } from 'vitest';
 import { encode, Replacer, encodeWithSelfDescribedTag } from './encode';
+import { decode } from '../decode/decode';
 import { CborValue } from '../cbor-value';
 
 function bytesToHexArray(arrayBuffer: Uint8Array): string[] {
@@ -346,3 +347,94 @@ describe('encodeWithSelfDescribedTag', () => {
     expect(bytesToHexString(results[2])).toEqual('D9D9F7A2616505616606'); // { "e": 5, "f": 6 } with self-described tag
   });
 });
+
+// Regression: encoder throws "DataView setUint8 offset out of bounds"
+// when a map value fills the 2KB buffer exactly, because the next key's
+// encodeTextString → encodeHeader has no bounds check.
+//
+// Byte layout for encodeWithSelfDescribedTag({ a: Uint8Array(2039), b: 'x' }):
+//   Tag 55799:        3 bytes  → s = 3
+//   Map(2):           1 byte   → s = 4
+//   TextString "a":   2 bytes  → s = 6
+//   ByteString(2039): 3 + 2039 → s = 2048 (= initial buffer length)
+//   TextString "b":   encodeHeader calls setUint8(2048, ...) → RangeError
+//
+// The encoder's internal buffer is module-level state that grows but never
+// shrinks. Earlier tests (e.g. 16 MB string) expand it well past 2 KB, masking
+// this bug. Each test below resets the module registry so it gets a fresh 2 KB
+// buffer.
+describe('encoder buffer overflow (upstream @dfinity/cbor bug)', () => {
+  let freshEncode: typeof encode;
+  let freshEncodeWithSelfDescribedTag: typeof encodeWithSelfDescribedTag;
+  let freshDecode: typeof decode;
+
+  beforeEach(async () => {
+    vi.resetModules();
+    const enc = await import('./encode');
+    const dec = await import('../decode/decode');
+    freshEncode = enc.encode;
+    freshEncodeWithSelfDescribedTag = enc.encodeWithSelfDescribedTag;
+    freshDecode = dec.decode;
+  });
+
+  test('encodeWithSelfDescribedTag: value exactly fills 2KB buffer', () => {
+    const payload = { a: new Uint8Array(2039), b: 'x' };
+
+    expect(() => freshEncodeWithSelfDescribedTag(payload)).not.toThrow();
+    const decoded = freshDecode(freshEncodeWithSelfDescribedTag(payload)) as {
+      a: Uint8Array;
+      b: string;
+    };
+    expect(decoded.b).toBe('x');
+    expect(decoded.a.byteLength).toBe(2039);
+  });
+
+  test('encode (no self-described tag): value exactly fills 2KB buffer', () => {
+    // Without the 3-byte tag prefix, the boundary shifts:
+    //   Map(2): 1 byte → s = 1, TextString "a": 2 bytes → s = 3,
+    //   ByteString(2042): 3 + 2042 → s = 2048
+    const payload = { a: new Uint8Array(2042), b: 'x' };
+
+    expect(() => freshEncode(payload)).not.toThrow();
+    const decoded = freshDecode(freshEncode(payload)) as {
+      a: Uint8Array;
+      b: string;
+    };
+    expect(decoded.b).toBe('x');
+    expect(decoded.a.byteLength).toBe(2042);
+  });
+
+  test('envelope-shaped payload with large arg and delegation', () => {
+    const envelope = {
+      content: {
+        request_type: 'call',
+        canister_id: new Uint8Array(10),
+        method_name: 'manage_neuron',
+        arg: new Uint8Array(1961),
+        sender: new Uint8Array(29),
+        ingress_expiry: BigInt('0x17E83B35D5A00000'),
+        nonce: new Uint8Array(16),
+      },
+      sender_sig: new Uint8Array(64),
+      sender_delegation: [
+        {
+          delegation: {
+            pubkey: new Uint8Array(93),
+            expiration: BigInt('0x17E83B35D5A00000'),
+          },
+          signature: new Uint8Array(1251),
+        },
+      ],
+      sender_pubkey: new Uint8Array(62),
+    };
+
+    expect(() => freshEncodeWithSelfDescribedTag(envelope)).not.toThrow();
+    const decoded = freshDecode(
+      freshEncodeWithSelfDescribedTag(envelope),
+    ) as Record<string, unknown>;
+    expect(decoded).toHaveProperty('content');
+    expect(decoded).toHaveProperty('sender_sig');
+    expect(decoded).toHaveProperty('sender_delegation');
+    expect(decoded).toHaveProperty('sender_pubkey');
+  });
+});
diff --git a/src/encode/encode.ts b/src/encode/encode.ts
index 4e9b38c..014740a 100644
--- a/src/encode/encode.ts
+++ b/src/encode/encode.ts
@@ -97,12 +97,16 @@ export function encodeWithSelfDescribedTag<T = any>(
   return target.slice(0, bytesOffset);
 }
 
-function encodeItem(item: CborValue, replacer?: Replacer): void {
-  if (bytesOffset > target.length - SAFE_BUFFER_END_OFFSET) {
-    target = resizeUint8Array(target, target.length * 2);
-    targetView = new DataView(target.buffer);
+function growBuffer(minSize: number): void {
+  let newSize = target.length * 2;
+  while (newSize < minSize) {
+    newSize *= 2;
   }
+  target = resizeUint8Array(target, newSize);
+  targetView = new DataView(target.buffer);
+}
 
+function encodeItem(item: CborValue, replacer?: Replacer): void {
   if (item === false || item === true || item === null || item === undefined) {
     encodeSimple(item);
     return;
@@ -161,6 +165,10 @@ function encodeMap(map: CborMap, replacer?: Replacer): void {
 }
 
 function encodeHeader(majorType: CborMajorType, value: CborNumber): void {
+  if (bytesOffset > target.length - SAFE_BUFFER_END_OFFSET) {
+    growBuffer(bytesOffset + SAFE_BUFFER_END_OFFSET);
+  }
+
   if (value <= TOKEN_VALUE_MAX) {
     targetView.setUint8(
       bytesOffset++,
@@ -240,8 +248,7 @@ function encodeBytes(majorType: CborMajorType, value: Uint8Array): void {
   encodeHeader(majorType, value.length);
 
   if (bytesOffset > target.length - value.length) {
-    target = resizeUint8Array(target, target.length + value.length);
-    targetView = new DataView(target.buffer);
+    growBuffer(bytesOffset + value.length);
   }
   target.set(value, bytesOffset);
   bytesOffset += value.length;
diff --git a/tsconfig.test.json b/tsconfig.test.json
index 7f50e7b..20bfb2c 100644
--- a/tsconfig.test.json
+++ b/tsconfig.test.json
@@ -1,6 +1,7 @@
 {
   "extends": "./tsconfig.json",
   "compilerOptions": {
+    "module": "ESNext",
     "moduleResolution": "bundler"
   },
   "include": ["./src/**/*", "./src/benchmarks/**/*", "./src/**/*.spec.ts"],