diff --git a/contracts/open-zeppelin/InitializableAdminUpgradeabilityProxy.sol b/contracts/open-zeppelin/InitializableAdminUpgradeabilityProxy.sol
index bc34f46..ac3ade8 100644
--- a/contracts/open-zeppelin/InitializableAdminUpgradeabilityProxy.sol
+++ b/contracts/open-zeppelin/InitializableAdminUpgradeabilityProxy.sol
@@ -20,7 +20,7 @@ contract InitializableAdminUpgradeabilityProxy is BaseAdminUpgradeabilityProxy,
    * This parameter is optional, if no data is given the initialization call to proxied contract will be skipped.
    */
     function initialize(address _logic, address _admin, bytes memory _data) public payable {
-        require(_implementation() == address(0));
+        require(_implementation() == address(0), "InitializableAdminUpgradeabilityProxy: already initialized");
         InitializableUpgradeabilityProxy.initialize(_logic, _data);
         assert(ADMIN_SLOT == bytes32(uint256(keccak256("eip1967.proxy.admin")) - 1));
         _setAdmin(_admin);