square connect api webhook update error

I'm getting

{"type":"bad_request","message":"Unexpected token }"}

from Update Webhooks endpoint. I'm trying to implement this in asp.net. My code block is following:

        var httpCLient = new HttpClient();
        httpCLient.BaseAddress = new Uri("https://connect.squareup.com");
        AuthenticationHeaderValue auth = new AuthenticationHeaderValue("Bearer", "access_token");
        httpCLient.DefaultRequestHeaders.Authorization = auth;
        httpCLient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
        HttpContent content = new StringContent("{\"PAYMENT_UPDATED\"}", System.Text.Encoding.UTF8, "application/json");
        Task<HttpResponseMessage> httpResponse = httpCLient.PutAsync("/v1/me/webhooks", content);
        if (httpResponse.Result.StatusCode == HttpStatusCode.OK)
        {
        }

Can anyone help me with this? I've already searched for any connect API implementation guide/example on Google, but no luck.

Solution

This error is occurring because the list of requested event types must be a JSON array, not a JSON object.

The request should succeed if you change {\"PAYMENT_UPDATED\"} to [\"PAYMENT_UPDATED\"] in the above example.