UniswapV2 liquidity pool reserves return 0 for specific addresses

Question

I have a contract which tries to calculate the token price based on the reserves in the liquidity pools. When I call the contract function getTokenPriceAB() and input the pair address of WBTC-USDC as a parameter, it successfully returns the amount.

But when I change it to another one like WBTC-WETH, it returns 0.

   function getTokenPriceAB(address pairAddress) public override view returns(uint) {

        IUniswapV2Pair pair = IUniswapV2Pair(pairAddress);

        IERC20 token1 = IERC20(pair.token1());
        (uint Res0, uint Res1,) = pair.getReserves();

        require(Res1 != 0, "Division by zero");

        // decimals
        uint res0 = Res0.mul(10**token1.decimals());

        // return amount of _tokenA needed to buy _tokenB
        // to improve precision, multiply before dividing
        return((1000000000000000000*res0)/Res1); 
    }

I am expecting any other result than 0 independent from the pair address.

I also made sure that I input a valid address in there and checked that this pool has actually liquidity in it on etherscan.

Answer 1

Stop using getReserves(), use kLast()

   function getTokenPriceAB(address pairAddress) public view returns(uint256) {

        IUniswapV2Pair pair = IUniswapV2Pair(pairAddress);
        uint256 price = pair.kLast();
        return price;

    }

Using the kLast() function instead of getReserves() makes sense in this context because it returns the last trade price of the token pair. Rather than the current reserves of the token pair, it returns the last trade price of a token pair, which is calculated as the ratio of the token0 price to token1 price.

Answer 2

I tried using kLast() on the WETH-USDC pair on etherscan but even the read contract function returns always 0 for kLast(). So I am at least correctly calling the function. Why is the value always 0 ? The liquidity pool has clearly funds in it so it should be able to calculate the kLast price.

Also checking the getReserves() function, it successfully returns both values.

This doesn't make sense at all and I can't find any information in the documentation.