diff --git a/contracts/SecurityTokenRegistry.sol b/contracts/SecurityTokenRegistry.sol
index f16baab30..cac8fcf2f 100644
--- a/contracts/SecurityTokenRegistry.sol
+++ b/contracts/SecurityTokenRegistry.sol
@@ -831,6 +831,8 @@ contract SecurityTokenRegistry is EternalStorage, Proxy {
     function _setProtocolFactory(address _STFactoryAddress, uint8 _major, uint8 _minor, uint8 _patch) internal {
         require(_STFactoryAddress != address(0), "Bad address");
         uint24 _packedVersion = VersionUtils.pack(_major, _minor, _patch);
+        address stFactoryAddress = getAddressValue(Encoder.getKey("protocolVersionST", uint256(_packedVersion)));
+        require(stFactoryAddress == address(0), "Already exists");
         set(Encoder.getKey("protocolVersionST", uint256(_packedVersion)), _STFactoryAddress);
         emit ProtocolFactorySet(_STFactoryAddress, _major, _minor, _patch);
     }
diff --git a/test/n_security_token_registry.js b/test/n_security_token_registry.js
index 2629f098c..2ac076d6c 100644
--- a/test/n_security_token_registry.js
+++ b/test/n_security_token_registry.js
@@ -684,7 +684,10 @@ contract("SecurityTokenRegistry", async (accounts) => {
             assert.equal(_protocol[0], 2);
             assert.equal(_protocol[1], 2);
             assert.equal(_protocol[2], 0);
-            await I_STRProxied.setProtocolFactory(I_STFactory.address, new BN(3), new BN(0), new BN(0), { from: account_polymath});
+            await catchRevert(
+                I_STRProxied.setProtocolFactory(I_STFactory.address, new BN(3), new BN(0), new BN(0), { from: account_polymath})
+            );
+            await I_STRProxied.setProtocolFactory(I_STFactory.address, new BN(3), new BN(0), new BN(1), { from: account_polymath});
             await I_STRProxied.setLatestVersion(new BN(3), new BN(0), new BN(0), { from: account_polymath});
             _protocol = await I_Getter.getLatestProtocolVersion.call();
             assert.equal(_protocol[0], 3);