diff --git a/contracts/ERC20Mixer.sol b/contracts/ERC20Mixer.sol
index 0a03c4d..a7c6c58 100644
--- a/contracts/ERC20Mixer.sol
+++ b/contracts/ERC20Mixer.sol
@@ -17,7 +17,7 @@ contract ERC20Mixer is Mixer {
   address public token;
 
   constructor(
-    address _verifier,
+    IVerifier _verifier,
     uint256 _denomination,
     uint8 _merkleTreeHeight,
     uint256 _emptyElement,
diff --git a/contracts/ETHMixer.sol b/contracts/ETHMixer.sol
index b575ce5..56a9aed 100644
--- a/contracts/ETHMixer.sol
+++ b/contracts/ETHMixer.sol
@@ -15,7 +15,7 @@ import "./Mixer.sol";
 
 contract ETHMixer is Mixer {
   constructor(
-    address _verifier,
+    IVerifier _verifier,
     uint256 _denomination,
     uint8 _merkleTreeHeight,
     uint256 _emptyElement,
diff --git a/contracts/Mixer.sol b/contracts/Mixer.sol
index 11a0ad3..b16383c 100644
--- a/contracts/Mixer.sol
+++ b/contracts/Mixer.sol
@@ -47,13 +47,13 @@ contract Mixer is MerkleTreeWithHistory {
     @param _operator operator address (see operator above)
   */
   constructor(
-    address _verifier,
+    IVerifier _verifier,
     uint256 _denomination,
     uint8 _merkleTreeHeight,
     uint256 _emptyElement,
     address _operator
   ) MerkleTreeWithHistory(_merkleTreeHeight, _emptyElement) public {
-    verifier = IVerifier(_verifier);
+    verifier = _verifier;
     operator = _operator;
     denomination = _denomination;
   }