POST action results in "Invalid http header"

I am trying to execute a GET command so I get can get data from a server. The Curl below works in Postman.

$ curl -X GET -H "Authorization: {SESSION_ID}"
https://{server}/api/{version}/metadata/vobjects
While running my code, I am able to get the session ID. The next step is to get the data. But when I execute the GET, I don't get any response. Instead a I get an error as follows: "Specified value has invalid HTTP Header characters. (Parameter 'name')"

Below is the C# code I am trying to execute

_

public static void getObjects(string sessionID)
{
var client = new RestClient("https://generic-server.com/api/v20.3/metadata/vobjects/");
client.Timeout = -1;
var request = new RestRequest(Method.GET);
request.AddHeader("Authorization:", sessionID);
IRestResponse response = client.Execute(request);
Console.WriteLine("Here's the response: " + response.Content + " " + response.Content.Length);
Console.WriteLine("Here's the error message: " + response.ErrorMessage);
}

_

response.ErrorMessage gives the following:

Here's the error message: Specified value has invalid HTTP Header characters. (Parameter 'name')

Note: When I leave the colon out of of the header, then I get the following response from the server:

{"responseStatus":"FAILURE","errors":[{"type":"INVALID_DATA","message":"Invalid http header [Accept: application/json, text/json, text/x-json, text/javascript, application/xml, text/xml]"}]} 190

Does anyone have any thoughts

Start by reproducing the request shown in the C# getObjects method and compare it with the working curl command. Confirm the intended request-header and response behavior against the API; the issue is done when the GET returns the requested metadata without header errors.