diff --git a/contracts/DATAv2.sol b/contracts/DATAv2.sol
index 12bceb2..4f0ec90 100644
--- a/contracts/DATAv2.sol
+++ b/contracts/DATAv2.sol
@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity >=0.8.0;
+pragma solidity 0.8.6;
 
 import "@openzeppelin/contracts/access/AccessControl.sol";
 import "@openzeppelin/contracts/token/ERC20/extensions/ERC20Burnable.sol";
diff --git a/contracts/DataTokenMigrator.sol b/contracts/DataTokenMigrator.sol
index 6dfc2e0..a78878a 100644
--- a/contracts/DataTokenMigrator.sol
+++ b/contracts/DataTokenMigrator.sol
@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity >=0.8.0;
+pragma solidity 0.8.6;
 
 import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
 
@@ -30,7 +30,7 @@ contract DataTokenMigrator { // is UpgradeAgent, see Crowdsale.sol
      * @param _from token holder that called CrowdsaleToken.upgrade(_value)
      * @param _value amount of tokens to upgrade, checked by the CrowdsaleToken
      */
-    function upgradeFrom(address _from, uint256 _value) public {
+    function upgradeFrom(address _from, uint256 _value) external {
         require(
             msg.sender == address(oldToken),
             "Call not permitted, UpgradableToken only"
diff --git a/contracts/IERC677.sol b/contracts/IERC677.sol
index d231694..f484885 100644
--- a/contracts/IERC677.sol
+++ b/contracts/IERC677.sol
@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity 0.8.6;
 
 import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
 
diff --git a/contracts/IERC677Receiver.sol b/contracts/IERC677Receiver.sol
index 6e3e7da..d88064f 100644
--- a/contracts/IERC677Receiver.sol
+++ b/contracts/IERC677Receiver.sol
@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity 0.8.6;
 
 interface IERC677Receiver {
     function onTokenTransfer(
diff --git a/contracts/MockRecipient.sol b/contracts/MockRecipient.sol
index a6e6885..69766f6 100644
--- a/contracts/MockRecipient.sol
+++ b/contracts/MockRecipient.sol
@@ -1,5 +1,5 @@
 //SPDX-License-Identifier: Unlicense
-pragma solidity ^0.8.0;
+pragma solidity 0.8.6;
 
 import "hardhat/console.sol";
 import "./IERC677Receiver.sol";
diff --git a/hardhat.config.js b/hardhat.config.js
index d57a923..f76c85c 100644
--- a/hardhat.config.js
+++ b/hardhat.config.js
@@ -25,7 +25,7 @@ module.exports = {
     solidity: {
         compilers: [
             {
-                version: "0.8.3",
+                version: "0.8.6",
             },
             {
                 version: "0.4.11"