Want to solve Uniswap's puzzle

[formysister]

In UniswapV2Router original code, we can find the method named pairFor().
This method is called when liquidity added to get pair.

But when deploy smart contract with this original method, addLiquidity() method makes error and transaction is reverted.

So all customers need to redefine the functions like below.

function pairFor(address factory, address tokenA, address tokenB) internal view returns (address pair) {
        pair = IUniswapV2Factory(factory).getPair(tokenA,tokenB);
}

What's the wrong with that function and how original Uniswap deployed initially?