diff --git a/.env.example b/.env.example
index 90d859c..e7cef8f 100644
--- a/.env.example
+++ b/.env.example
@@ -1,6 +1,6 @@
 NET_ID=42
 RPC_URL=https://kovan.infura.io/v3/a3f4d001c1fc4a359ea70dd27fd9cb51
 PRIVATE_KEY=
-MIXER_ADDRESS=0xa01dCfABF44674f2f60e064e9F4B1300a87753c8
+MIXER_ADDRESS=0x5375776Da34366b9A4C8a44F6303B44b5Ee2F07a
 # in wei
 DESIRED_FEE=10000000000000000
\ No newline at end of file
diff --git a/config.js b/config.js
index 88f2130..0348498 100644
--- a/config.js
+++ b/config.js
@@ -6,6 +6,6 @@ module.exports = {
   privateKey: process.env.PRIVATE_KEY,
   mixerAddress: process.env.MIXER_ADDRESS,
   desiredFee: process.env.DESIRED_FEE || 10000000000000000, // 0.01 ETH
-  defaultGasPrice: 1,
+  defaultGasPrice: 2,
   gasOracleUrls: ['https://www.etherchain.org/api/gasPriceOracle', 'https://gasprice.poa.network/']
 }
\ No newline at end of file
diff --git a/index.js b/index.js
index dc079f6..812cf14 100644
--- a/index.js
+++ b/index.js
@@ -10,7 +10,7 @@ app.use(function(req, res, next) {
   next()
 })
 
-const { netId, rpcUrl, privateKey, mixerAddress, defaultGasPrice, desiredFee } = require('./config')
+const { netId, rpcUrl, privateKey, mixerAddress, defaultGasPrice } = require('./config')
 const { fetchGasPrice, isValidProof } = require('./utils')
 
 const web3 = new Web3(rpcUrl, null, { transactionConfirmationBlocks: 1 })
@@ -31,27 +31,28 @@ app.post('/relay', async (req, resp) => {
   const { valid , reason } = isValidProof(req.body)
   if (!valid) {
     console.log('Proof is invalid:', reason)
-    return resp.status(400).send('Proof is invalid')
+    return resp.status(400).json({ error: 'Proof is invalid' })
   }
 
   let { pi_a, pi_b, pi_c, publicSignals } = req.body
 
   const fee = toBN(publicSignals[3])
-  if (fee.lt(toBN(desiredFee))) {
+  const desiredFee = toBN(toWei(gasPrices.fast.toString(), 'gwei')).mul(toBN('1000000'))
+  if (fee.lt(desiredFee)) {
     console.log('Fee is too low')
-    return resp.status(400).send('Fee is too low')
+    return resp.status(400).json({ error: 'Fee is too low. Try to resend.' })
   }
 
   try {
     const nullifier = publicSignals[1]
     const isSpent = await mixer.methods.isSpent(nullifier).call()
     if (isSpent) {
-      throw new Error('The note has been spent')
+      return resp.status(400).json({ error: 'The note has been spent.' })
     }
     const root = publicSignals[0]
     const isKnownRoot = await mixer.methods.isKnownRoot(root).call()
     if (!isKnownRoot) {
-      throw new Error('The merkle root is too old or invalid')
+      return resp.status(400).json({ error: 'The merkle root is too old or invalid.' })
     }
     const gas = await mixer.methods.withdraw(pi_a, pi_b, pi_c, publicSignals).estimateGas()
     const result = mixer.methods.withdraw(pi_a, pi_b, pi_c, publicSignals).send({
@@ -63,11 +64,11 @@ app.post('/relay', async (req, resp) => {
       resp.json({ txHash: hash })
     }).on('error', function(e){
       console.log(e)
-      resp.status(400).send('Proof is malformed')
+      return resp.status(400).json({ error: 'Proof is malformed.' })
     })
   } catch (e) {
     console.log(e)
-    resp.status(400).send('Proof is malformed or spent')
+    return resp.status(400).json({ error: 'Proof is malformed or spent.' })
   }
 })