Truffle constructor type omits JSON allowing access to contract name

I have some code which attempts to get the contract name by checking `contractInstance.constructor._json.contractName` and this executes fine, but can't compile with TypeChain, throwing error `Property '_json' does not exist on type 'Function'.ts(2339)`. Is there a better way to get the contract name, or can the typings be added to allow access to the `_json` property?