diff --git a/34_ERC721/Address.sol b/34_ERC721/Address.sol
deleted file mode 100644
index 0861a44c..00000000
--- a/34_ERC721/Address.sol
+++ /dev/null
@@ -1,14 +0,0 @@
-// SPDX-License-Identifier: MIT
-pragma solidity ^0.8.1;
-
-// Address库
-library Address {
-    // 利用extcodesize判断一个地址是否为合约地址
-    function isContract(address account) internal view returns (bool) {
-        uint size;
-        assembly {
-            size := extcodesize(account)
-        }
-        return size > 0;
-    }
-}
diff --git a/34_ERC721/ERC721.sol b/34_ERC721/ERC721.sol
index 82492819..d43b3351 100644
--- a/34_ERC721/ERC721.sol
+++ b/34_ERC721/ERC721.sol
@@ -6,11 +6,9 @@ import "./IERC165.sol";
 import "./IERC721.sol";
 import "./IERC721Receiver.sol";
 import "./IERC721Metadata.sol";
-import "./Address.sol";
 import "./String.sol";
 
 contract ERC721 is IERC721, IERC721Metadata{
-    using Address for address; // 使用Address库，用isContract来判断地址是否为合约
     using Strings for uint256; // 使用String库，
 
     // Token名称
@@ -26,6 +24,9 @@ contract ERC721 is IERC721, IERC721Metadata{
     //  owner地址。到operator地址 的批量授权映射
     mapping(address => mapping(address => bool)) private _operatorApprovals;
 
+    // 错误 无效的接收者
+    error ERC721InvalidReceiver(address receiver);
+
     /**
      * 构造函数，初始化`name` 和`symbol` .
      */
@@ -165,7 +166,7 @@ contract ERC721 is IERC721, IERC721Metadata{
         bytes memory _data
     ) private {
         _transfer(owner, from, to, tokenId);
-        require(_checkOnERC721Received(from, to, tokenId, _data), "not ERC721Receiver");
+        _checkOnERC721Received(from, to, tokenId, _data);
     }
 
     /**
@@ -225,22 +226,22 @@ contract ERC721 is IERC721, IERC721Metadata{
     }
 
     // _checkOnERC721Received：函数，用于在 to 为合约的时候调用IERC721Receiver-onERC721Received, 以防 tokenId 被不小心转入黑洞。
-    function _checkOnERC721Received(
-        address from,
-        address to,
-        uint tokenId,
-        bytes memory _data
-    ) private returns (bool) {
-        if (to.isContract()) {
-            return
-                IERC721Receiver(to).onERC721Received(
-                    msg.sender,
-                    from,
-                    tokenId,
-                    _data
-                ) == IERC721Receiver.onERC721Received.selector;
-        } else {
-            return true;
+    function _checkOnERC721Received(address from, address to, uint256 tokenId, bytes memory data) private {
+        if (to.code.length > 0) {
+            try IERC721Receiver(to).onERC721Received(msg.sender, from, tokenId, data) returns (bytes4 retval) {
+                if (retval != IERC721Receiver.onERC721Received.selector) {
+                    revert ERC721InvalidReceiver(to);
+                }
+            } catch (bytes memory reason) {
+                if (reason.length == 0) {
+                    revert ERC721InvalidReceiver(to);
+                } else {
+                    /// @solidity memory-safe-assembly
+                    assembly {
+                        revert(add(32, reason), mload(reason))
+                    }
+                }
+            }
         }
     }
 
diff --git a/34_ERC721/readme.md b/34_ERC721/readme.md
index 54b960dd..49d07071 100644
--- a/34_ERC721/readme.md
+++ b/34_ERC721/readme.md
@@ -156,7 +156,7 @@ interface IERC721Receiver {
         uint tokenId,
         bytes memory _data
     ) private returns (bool) {
-        if (to.isContract()) {
+        if (to.code.length > 0) {
             return
                 IERC721Receiver(to).onERC721Received(
                     msg.sender,
@@ -199,11 +199,9 @@ import "./IERC165.sol";
 import "./IERC721.sol";
 import "./IERC721Receiver.sol";
 import "./IERC721Metadata.sol";
-import "./Address.sol";
 import "./String.sol";
 
 contract ERC721 is IERC721, IERC721Metadata{
-    using Address for address; // 使用Address库，用isContract来判断地址是否为合约
     using Strings for uint256; // 使用String库，
 
     // Token名称
@@ -219,6 +217,9 @@ contract ERC721 is IERC721, IERC721Metadata{
     //  owner地址。到operator地址 的批量授权映射
     mapping(address => mapping(address => bool)) private _operatorApprovals;
 
+    // 错误 无效的接收者
+    error ERC721InvalidReceiver(address receiver);
+
     /**
      * 构造函数，初始化`name` 和`symbol` .
      */
@@ -294,7 +295,7 @@ contract ERC721 is IERC721, IERC721Metadata{
         _approve(owner, to, tokenId);
     }
 
-    // 查询 spender地址是否可以使用tokenId（他是owner或被授权地址）。
+    // 查询 spender地址是否可以使用tokenId（需要是owner或被授权地址）
     function _isApprovedOrOwner(
         address owner,
         address spender,
@@ -358,7 +359,7 @@ contract ERC721 is IERC721, IERC721Metadata{
         bytes memory _data
     ) private {
         _transfer(owner, from, to, tokenId);
-        require(_checkOnERC721Received(from, to, tokenId, _data), "not ERC721Receiver");
+        _checkOnERC721Received(from, to, tokenId, _data);
     }
 
     /**
@@ -418,22 +419,22 @@ contract ERC721 is IERC721, IERC721Metadata{
     }
 
     // _checkOnERC721Received：函数，用于在 to 为合约的时候调用IERC721Receiver-onERC721Received, 以防 tokenId 被不小心转入黑洞。
-    function _checkOnERC721Received(
-        address from,
-        address to,
-        uint tokenId,
-        bytes memory _data
-    ) private returns (bool) {
-        if (to.isContract()) {
-            return
-                IERC721Receiver(to).onERC721Received(
-                    msg.sender,
-                    from,
-                    tokenId,
-                    _data
-                ) == IERC721Receiver.onERC721Received.selector;
-        } else {
-            return true;
+    function _checkOnERC721Received(address from, address to, uint256 tokenId, bytes memory data) private {
+        if (to.code.length > 0) {
+            try IERC721Receiver(to).onERC721Received(msg.sender, from, tokenId, data) returns (bytes4 retval) {
+                if (retval != IERC721Receiver.onERC721Received.selector) {
+                    revert ERC721InvalidReceiver(to);
+                }
+            } catch (bytes memory reason) {
+                if (reason.length == 0) {
+                    revert ERC721InvalidReceiver(to);
+                } else {
+                    /// @solidity memory-safe-assembly
+                    assembly {
+                        revert(add(32, reason), mload(reason))
+                    }
+                }
+            }
         }
     }
 
@@ -455,6 +456,7 @@ contract ERC721 is IERC721, IERC721Metadata{
         return "";
     }
 }
+
 ```
 
 ## 写一个免费铸造的APE