ambiguous primary types or unused types: "EIP712Domain", "Mail"

The listed names are your unreferenced types. `err.message` adds the full `types` value and the ethers version; `err.code` is `INVALID_ARGUMENT`.

ethers v6 (JavaScript) — TypedDataEncoder used by signTypedData / verifyTypedData

What it means

ethers infers the primary type of EIP-712 typed data as the single type that no other type references. In your `types` object several types are unreferenced, so it cannot tell which one is being signed — and in practice the extra one is almost always `EIP712Domain`. ethers builds the domain type itself from the `domain` argument and must not receive it in `types`, while the `eth_signTypedData_v4` JSON format (MetaMask, WalletConnect) includes it.

Why it happens

• EIP712Domain left in types

  common

  Typed data copied from a wallet request, a back end that produces `eth_signTypedData_v4` payloads, or a test fixture includes `EIP712Domain`. ethers treats it as a second root type.

• A type that nothing references

  occasional

  Leftover struct definitions (for example from an older version of the message) are roots too and make the primary type ambiguous.

• Two independent structs in one object

  occasional

  Types for different messages (Order and Cancel) combined into one `types` object. Pass only the types of the message you sign.

How to fix it

1. 1.

   Remove EIP712Domain before calling ethers

   Destructure it away; the signature is identical to the one a wallet produces for the v4 payload, because the domain type is derived from the same fields.

   const { EIP712Domain, ...types } = typedData.types; // ethers derives the domain type itself
   const signature = await signer.signTypedData(typedData.domain, types, typedData.message);
   const signer2 = verifyTypedData(typedData.domain, types, typedData.message, signature);

2. 2.

   Check the inferred primary type

   `TypedDataEncoder.from(types).primaryType` throws the same error or returns the type ethers will hash — a quick check in tests.

   import { TypedDataEncoder } from "ethers";
   
   // Inspect what ethers considers the primary type before signing.
   console.log(TypedDataEncoder.from(types).primaryType);

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