# Troubleshooting (v2) (/docs/protocols/v2/guides/troubleshooting)

Diagnose common Uniswap v2 integration errors, including K, rebasing edge cases, and router transaction failures.

This page covers error codes and failure patterns frequently encountered while integrating with Uniswap v2.

## UniswapV2: K
`K` errors happen when a swap attempt leaves pair reserves below invariant expectations.

## Fee On Transfer Tokens
## Inclusive fee-on-transfer tokens
These tokens burn or divert part of each transfer, so the recipient receives less than the sender transfers.

Use router functions ending in `SupportingFeeOnTransfer` to validate output amounts on receipt.

## Exclusive fee-on-transfer tokens
These tokens send an additional trailing transfer after the primary transfer.

Because the router cannot anticipate that trailing transfer, swaps may revert or leave pair state inconsistent.

## Rebasing Tokens
Rebasing tokens can change balances independently of transfers, which can create reserve drift.

## Negative rebasing tokens
Negative rebases can unbalance pool reserves and shift losses to the next transacting account.

## Positive rebasing tokens
Positive rebases can create surplus balances that are not reflected in reserve accounting. Third parties may capture this surplus via `skim()`.

## Notes on rebasing support
If you are designing rebasing tokens, account for integration behavior explicitly. Wrapper patterns such as [CHAI](https://chai.money/about.html) can improve compatibility.

## Router and Transaction Errors
## UniswapV2: LOCKED
`LOCKED` is the router reentrancy guard.

## No Access To Archive Node
This is usually an environment issue with local fork tooling.

UniswapV2: TRANSFER_FAILED [#uniswapv2-transfer_failed]

The core contract could not transfer tokens to the recipient.

## UniswapV2: EXPIRED
The transaction deadline elapsed before inclusion onchain.

## Action Requires an Active Reserve
`VM Exception While Processing Transaction: Action Requires an Active Reserve`

## Unable To Approve Transaction On The Front End
Some tokens block direct allowance increases. Setting allowance to zero first, then setting the target amount, usually resolves this.