diff --git a/packages/hardhat/contracts/SubscryptoToken.sol b/packages/hardhat/contracts/SubscryptoToken.sol
index 286be40..06c62c5 100644
--- a/packages/hardhat/contracts/SubscryptoToken.sol
+++ b/packages/hardhat/contracts/SubscryptoToken.sol
@@ -279,6 +279,7 @@ contract SubscryptoToken is ERC20, ERC20Burnable, Ownable, ERC20Permit {
 		address customer,
 		uint tierIndex
 	) private {
+		require(tierIndex < tiers[merchant].length, 'No such tier offered by this merchant.');
 		accountAtSubscriptionEnd(merchant, customer);
 		subscriptions[merchant][customer] = Subscription({
 			tier: tierIndex,