diff --git a/contracts/MerkleTreeWithHistory.sol b/contracts/MerkleTreeWithHistory.sol
index 9d2f189..39145b9 100644
--- a/contracts/MerkleTreeWithHistory.sol
+++ b/contracts/MerkleTreeWithHistory.sol
@@ -56,7 +56,7 @@ contract MerkleTreeWithHistory {
     hash = R;
   }
 
-  function _insert(uint256 leaf) internal {
+  function _insert(uint256 leaf) internal returns(uint256 index) {
     uint32 current_index = next_index;
     require(current_index != 2**levels, "Merkle tree is full. No more leafs can be added");
     next_index += 1;
@@ -82,6 +82,7 @@ contract MerkleTreeWithHistory {
 
     current_root = (current_root + 1) % ROOT_HISTORY_SIZE;
     _roots[current_root] = current_level_hash;
+    return next_index - 1;
   }
 
   function isKnownRoot(uint256 root) public view returns(bool) {
diff --git a/contracts/Mixer.sol b/contracts/Mixer.sol
index 4f273ad..8da900e 100644
--- a/contracts/Mixer.sol
+++ b/contracts/Mixer.sol
@@ -65,11 +65,11 @@ contract Mixer is MerkleTreeWithHistory {
   function deposit(uint256 commitment) public payable {
     require(isDepositsEnabled, "deposits are disabled");
     require(!commitments[commitment], "The commitment has been submitted");
-    _insert(commitment);
+    uint256 insertedIndex = _insert(commitment);
     commitments[commitment] = true;
     _processDeposit();
 
-    emit Deposit(commitment, next_index - 1, block.timestamp);
+    emit Deposit(commitment, insertedIndex, block.timestamp);
   }
 
   /** @dev this function is defined in a child contract */