diff --git a/contracts/accounts/Simple7702Account.sol b/contracts/accounts/Simple7702Account.sol
index 74d960862..91489dbcf 100644
--- a/contracts/accounts/Simple7702Account.sol
+++ b/contracts/accounts/Simple7702Account.sol
@@ -33,7 +33,6 @@ contract Simple7702Account is BaseAccount, IERC165, IERC1271, ERC1155Holder, ERC
         PackedUserOperation calldata userOp,
         bytes32 userOpHash
     ) internal virtual override returns (uint256 validationData) {
-
         return _checkSignature(userOpHash, userOp.signature) ? SIG_VALIDATION_SUCCESS : SIG_VALIDATION_FAILED;
     }
 
@@ -42,7 +41,7 @@ contract Simple7702Account is BaseAccount, IERC165, IERC1271, ERC1155Holder, ERC
     }
 
     function _checkSignature(bytes32 hash, bytes memory signature) internal view returns (bool) {
-        return ECDSA.recover(hash, signature) == address(this);
+        return signature.length == 65 && ECDSA.recover(hash, signature) == address(this);
     }
 
     function _requireForExecute() internal view virtual override {
diff --git a/contracts/accounts/SimpleAccount.sol b/contracts/accounts/SimpleAccount.sol
index 82ccebf43..8415af126 100644
--- a/contracts/accounts/SimpleAccount.sol
+++ b/contracts/accounts/SimpleAccount.sol
@@ -89,10 +89,14 @@ contract SimpleAccount is BaseAccount, TokenCallbackHandler, UUPSUpgradeable, In
     /// implement template method of BaseAccount
     function _validateSignature(PackedUserOperation calldata userOp, bytes32 userOpHash)
     internal override virtual returns (uint256 validationData) {
+        // bundler sets the signature as empty when estimating gas
+        if (userOp.signature.length != 65)
+            return SIG_VALIDATION_FAILED;
 
         // UserOpHash can be generated using eth_signTypedData_v4
         if (owner != ECDSA.recover(userOpHash, userOp.signature))
             return SIG_VALIDATION_FAILED;
+
         return SIG_VALIDATION_SUCCESS;
     }