Verifying signature for Smart Account

[GavinStein1]

I have a server side function that accepts a challenge and a signed challenge which it expects to be signed by a particular smart account. The function knows the address of the smart account (this is the address that is returned by the Smart Account client function getAccountAddress()).

To implement this functionality, I generate a challenge, and then get the user to sign this challenge. They do this with the following code in the frontend.

const signMessage = async (message: string) => {
    const signer = smartAccount.getSigner();
    const signed = await signer.signMessage(message);
    return signed;
}

(Note: smartAccount is type BiconomySmartAccountV2)

The challenge and signed challenge is then sent to the server. The server receives these parameters and hashes the challenge.
const challengeHash = ethers.hashMessage(challenge);

I then create an ethers.Contract object with the smartContract address, the ABI for a Biconomy smart account (from here), and an RPC url provider. On that contract object I call isValidSignature().

const smartAccountContract = new ethers.Contract(addr, abi, new ethers.JsonRpcProvider("https://sepolia.infura.io/v3/API_KEY"));
const returned = await smartAccountContract.isValidSignature(challengeHash, signedChallenge);

I thought this would work. It is how I've seen other people implement signature verification. Unfortunately I get the following error:

Error: missing revert data (action="call", data=null, reason=null, transaction={ "data": "0x1626ba7e50b2c43fd39106bafbba0da34fc430e1f91e3c96ea2acee2bc34119f92b377500000000000000000000000000000000000000000000000000000000000000040000000000000000000000000000000000000000000000000000000000000004164cfb7f977be25b5fcb294639487d2c06ca5c61ed3f7055fb4d7293effc0271c5c584fa1cbb07425c23c5c1cb383bab505c2acd22246f4898085df9a75d7fb571c00000000000000000000000000000000000000000000000000000000000000", "to": "[SMART_ACCOUNT_ADDRESS]" }

Does anyone have any idea what I am doing wrong?!