diff --git a/contracts/ERC20Permit.sol b/contracts/ERC20Permit.sol
index b3363be..1688eb3 100644
--- a/contracts/ERC20Permit.sol
+++ b/contracts/ERC20Permit.sol
@@ -31,8 +31,6 @@ contract ERC20Permit is IERC20Permit {
     // PERMIT_TYPEHASH = keccak256("Permit(address owner,address spender,uint256 amount,uint256 nonce,uint256 deadline)");
     bytes32 public constant override PERMIT_TYPEHASH = 0xfc77c2b9d30fe91687fd39abb7d16fcdfe1472d065740051ab8b13e4bf4a617f;
 
-    bytes32 public immutable override DOMAIN_SEPARATOR;
-
     mapping (address => uint256) public override nonces;
 
     /**
@@ -44,26 +42,24 @@ contract ERC20Permit is IERC20Permit {
         name     = name_;
         symbol   = symbol_;
         decimals = decimals_;
+    }
+
+    /**************************/
+    /*** External Functions ***/
+    /**************************/
 
-        uint256 chainId;
-        assembly {
-            chainId := chainid()
-        }
-        DOMAIN_SEPARATOR = keccak256(
+    function DOMAIN_SEPARATOR() public view override returns (bytes32 domainSeparator_) {
+        return keccak256(
             abi.encode(
                 keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"),
                 keccak256(bytes(name)),
                 keccak256(bytes("1")),
-                chainId,
+                block.chainid,
                 address(this)
             )
         );
     }
 
-    /**************************/
-    /*** External Functions ***/
-    /**************************/
-
     function approve(address spender_, uint256 amount_) external override returns (bool success_) {
         _approve(msg.sender, spender_, amount_);
         return true;
@@ -84,7 +80,7 @@ contract ERC20Permit is IERC20Permit {
         bytes32 digest = keccak256(
             abi.encodePacked(
                 "\x19\x01",
-                DOMAIN_SEPARATOR,
+                DOMAIN_SEPARATOR(),
                 keccak256(abi.encode(PERMIT_TYPEHASH, owner, spender, amount, nonces[owner]++, deadline))
             )
         );
diff --git a/contracts/interfaces/IERC20Permit.sol b/contracts/interfaces/IERC20Permit.sol
index 424abb3..608686b 100644
--- a/contracts/interfaces/IERC20Permit.sol
+++ b/contracts/interfaces/IERC20Permit.sol
@@ -25,16 +25,16 @@ interface IERC20Permit is IERC20 {
     function PERMIT_TYPEHASH() external pure returns (bytes32 hash_);
 
     /**
-     *  @dev    Returns the nonce for the given owner.
-     *  @param  owner The addreses of the owner account.
-     *  @return nonce_ The current nonce.
+      * @dev    Returns the nonce for the given owner.
+      * @param  owner The address of the owner account.
+      * @return nonce_ The current nonce.
      */
     function nonces(address owner) external view returns (uint256 nonce_);
 
     /**
-     *  @dev    Returns the signature domain separator.
-     *  @return domain_ The domain for the contract.
+     * @dev    Returns the signature domain separator.
+     * @return domainSeparator_ The domain for the contract.
      */
-    function DOMAIN_SEPARATOR() external view returns (bytes32 domain_);
+    function DOMAIN_SEPARATOR() external view returns (bytes32 domainSeparator_);
 
 }