> For the complete documentation index, see [llms.txt](/llms.txt).

# AA21 didn't pay prefund

The `EntryPoint` contract reverts with `AA21 didn't pay prefund` when a [smart account](/smart-accounts-kit/development/reference/glossary#metamask-smart-account)**MetaMask smart account** A smart contract account created using the Smart Accounts Kit that supports programmable behavior, flexible signing options, and ERC-7710 delegations. doesn't have enough native token balance to cover the gas cost of the [user operation](/smart-accounts-kit/development/reference/glossary#user-operation)**User operation** A pseudo-transaction object defined by ERC-4337 that describes what a smart account should execute. User operations are submitted to the alternate mempool managed by bundlers..

Before executing a user operation, the `EntryPoint` requires the sender account to prefund the expected gas cost. If the account's balance is lower than the required prefund, the `EntryPoint`reverts the operation.

## Solution[​](#solution "Direct link to Solution")

### Fund the smart account[​](#fund-the-smart-account "Direct link to Fund the smart account")

Fund the smart account with enough native tokens to cover the required prefund. Use Viem's [estimateUserOperationGas](https://viem.sh/account-abstraction/actions/bundler/estimateUserOperationGas)to get the gas estimates from your [bundler](/smart-accounts-kit/development/reference/glossary#bundler)**Bundler** An ERC-4337 component that manages the alternate mempool: it collects user operations from smart accounts, packages them, and submits them to the network., then calculate the required prefund based on the `EntryPoint` version.

- EntryPoint v0.7
- EntryPoint v0.6

```
import { formatEther } from 'viem'

const gasEstimate = await bundlerClient.estimateUserOperationGas({
  account: smartAccount,
  calls: [{ to: '0x...', value: 0n }],
})

const { maxFeePerGas } = await publicClient.estimateFeesPerGas()

const requiredGas =
  gasEstimate.verificationGasLimit +
  gasEstimate.callGasLimit +
  (gasEstimate.paymasterVerificationGasLimit ?? 0n) +
  (gasEstimate.paymasterPostOpGasLimit ?? 0n) +
  gasEstimate.preVerificationGas

const requiredPrefund = requiredGas * maxFeePerGas

const balance = await publicClient.getBalance({
  address: smartAccount.address,
})

if (balance < requiredPrefund) {
  console.log(
    `Insufficient balance: account has ${formatEther(balance)} ETH, ` +
      `but needs ${formatEther(requiredPrefund)} ETH`
  )
}

```

```
import { formatEther } from 'viem'

const gasEstimate = await bundlerClient.estimateUserOperationGas({
  account: smartAccount,
  calls: [{ to: '0x...', value: 0n }],
})

const { maxFeePerGas } = await publicClient.estimateFeesPerGas()

const requiredGas =
  gasEstimate.callGasLimit + gasEstimate.verificationGasLimit + gasEstimate.preVerificationGas

const requiredPrefund = requiredGas * maxFeePerGas

const balance = await publicClient.getBalance({
  address: smartAccount.address,
})

if (balance < requiredPrefund) {
  console.log(
    `Insufficient balance: account has ${formatEther(balance)} ETH, ` +
      `but needs ${formatEther(requiredPrefund)} ETH`
  )
}

```

### Use a paymaster[​](#use-a-paymaster "Direct link to Use a paymaster")

You can use a [paymaster](/smart-accounts-kit/development/reference/glossary#paymaster)**Paymaster** A service that pays for user operations on behalf of a smart account. to sponsor the gas fees for the smart account, so the account doesn't need to hold native tokens. For more information about configuring a paymaster, see [Send a gasless transaction](/smart-accounts-kit/development/guides/smart-accounts/send-gasless-transaction/).