2Captcha Api Cannot Set Property

I return from the API with the code below:

string result = SendCaptcha(txtApiKey.Text, txtSiteKey.Text, pageurl);
Thread.Sleep(10000);
string answer = GetAnswer(result, txtApiKey.Text);

I'm trying to post the answer I got in this way.

IJavaScriptExecutor js = (IJavaScriptExecutor)driver;
string title = (string)js.ExecuteScript("document.getElementById('g-captcha-response').innerHTML='" + answer + "';");

The error I got