diff --git a/contracts/lendingpool/LendingPool.sol b/contracts/lendingpool/LendingPool.sol
index fde9d760..1684247e 100644
--- a/contracts/lendingpool/LendingPool.sol
+++ b/contracts/lendingpool/LendingPool.sol
@@ -742,6 +742,8 @@ contract LendingPool is VersionedInitializable, ILendingPool, LendingPoolStorage
   ) external override {
     _whenNotPaused();
 
+    require(msg.sender == _reserves[asset].aTokenAddress, Errors.CALLER_MUST_BE_AN_ATOKEN);
+
     ValidationLogic.validateTransfer(
       from,
       _reserves,
@@ -759,7 +761,7 @@ contract LendingPool is VersionedInitializable, ILendingPool, LendingPoolStorage
         fromConfig.setUsingAsCollateral(reserveId, false);
       }
 
-      if (balanceToBefore == 0) {
+      if (balanceToBefore == 0 && amount != 0) {
         UserConfiguration.Map storage toConfig = _usersConfig[to];
         toConfig.setUsingAsCollateral(reserveId, true);
       }
diff --git a/contracts/libraries/helpers/Errors.sol b/contracts/libraries/helpers/Errors.sol
index 9ed7b07e..b515603f 100644
--- a/contracts/libraries/helpers/Errors.sol
+++ b/contracts/libraries/helpers/Errors.sol
@@ -46,6 +46,7 @@ library Errors {
   string public constant NO_MORE_RESERVES_ALLOWED = '59';
   string public constant INVALID_FLASH_LOAN_EXECUTOR_RETURN = '60';
   string public constant INCONSISTENT_FLASHLOAN_PARAMS = '69';
+  string public constant CALLER_MUST_BE_AN_ATOKEN = '76';
 
   // require error messages - aToken - DebtTokens
   string public constant CALLER_MUST_BE_LENDING_POOL = '28'; // 'The caller of this function must be a lending pool'