Constructor calling UniswapV2Factory.createPair reverts on Sepolia because the mainnet router address 0x7a250d... has no code on that chain

Symptom: an ERC-20 deploys fine until a Uniswap V2 pair creation is added to the constructor, after which deployment fails on a testnet with a bare Execution reverted and no reason string. Gas and account balance are adequate, so the money-related explanations do not apply.

uniswapV2Router = IUniswapV2Router02(0x7a250d5630B4cF539739dF2C5dAcb4c659F2488D); uniswapV2Pair = IUniswapV2Pair( IUniswapV2Factory(uniswapV2Router.factory()).createPair(address(this), uniswapV2Router.WETH()) );

Cause: that router address is the Ethereum mainnet deployment. Contract addresses are per-chain, and on Sepolia nothing is deployed there. A call to an address with no code does not revert on its own — it returns success with zero bytes — so the ABI decode of the expected address return from factory() fails and the whole constructor reverts with no message. The failure looks like a Uniswap problem when it is really "there is no contract at this address on this chain".

Fix: use the router deployed on the chain you are targeting. For Sepolia the answer cites 0xC532a74256D3Db42D0Bf7a0400fEFDbad7694008. Testnet deployments are not canonical and do get replaced, so verify on the block explorer that the address has code and that factory() and WETH() return sane values before deploying.

Generalise this: keep per-chain address constants in configuration rather than literals in Solidity, and when a call to an external protocol reverts with no reason on a new chain, check for code at the address first (eth_getCode, or cast code <addr>), before debugging your own logic. Doing pair creation in a constructor also makes the failure maximally opaque; moving it to a separate initialize call gives you a revert you can actually trace.

Source: adapted from a Stack Exchange answer by SYED ASAD KAZMI (question by SolidityNewbie), https://ethereum.stackexchange.com/questions/162080/execution-reverted-due-to-uniswapv2-pair-creation-in-constructor — CC BY-SA 4.0