Can remove liquidity manually but not with removeLiquidityETH, what am I doing wrong?

Question

I am developing a token using ganache/brownie. As the title says, if I try and use removeLiquidityETH to test removing liquidity tokens, for example like this

pair.approve(pair_address, lp_balance, {'from':accounts[0]})

router.removeLiquidityETH(my_token.address,lp_balance, 0,0, accounts[0], int(time.time())+60, {'from': accounts[0]})

Then I get an error IUniswapV2Router02.removeLiquidityETH confirmed (ds-math-sub-underflow)

and I do not see the ETH back in accounts[0].

However, looking at how removeLiquidityETH works I can implement each step manually

pair.approve(pair_address, lp_balance, {'from':accounts[0]})

pair.transferFrom(accounts[0], pair_address, lp_balance, {'from': pair_address})

tx = pair.burn(accounts[0], {'from': pair_address})
    
interface.IWETH(router.WETH()).withdraw(tx.events['Burn']['amount1'], {'from':accounts[0]})

And everything works as intended with my ETH and MyToken balances being transferred back to accounts[0].

I think the error is somewhere in approve or the transaction params {'from'} but I have tried every combination I can think of with pair.approve router.approve and my_token.approve and I still get the same error.

Does anyone have an example of how to use removeLiquidityETH properly? I have no issues with addLiquidityETH, in which I just use MyToken to approve the router spending.

Answer 1

From the docs

msg.sender should have already given the router an allowance of at least liquidity on the pool.

It doesn't seem like you're giving router an allowance to spend your liquidity tokens.

Answer 2

I assume that accounts[0] is the address holding the LP tokens. The issue seems to be that you are approving the pair contract as spender of the LP tokens whereas you should be approving the router as spender.

Replace this:

pair.approve(pair_address, balance, {'from':accounts[0]})

By this:

pair.approve(router_address, lp_balance, {'from':accounts[0]})