Troubleshooting (v2)

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 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

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.