get file size in ftp c#

I have to get the size of a file from ftp. I get this error

Unable to cast object of type 'System.Net.FileWebRequest' to type 
'System.Net.FtpWebRequest'

on the line I create the FtpWebRequest object. Here is the code I have at the moment:

public static long FtpGetFileSize(string uri, string username, string password)
{
        // Get the object used to communicate with the server

        FtpWebRequest request = (FtpWebRequest)WebRequest.Create(uri);
        request.Method = WebRequestMethods.Ftp.GetFileSize;

        // Get network credentials.
        request.Credentials = new NetworkCredential(username, password);

        try
        {
            using (FtpWebResponse response = (FtpWebResponse)request.GetResponse())
            {
                // Return the size.
                Console.WriteLine("El tamaño del archivo es {0}",response.ContentLength);
                return response.ContentLength;
            }
        }
        catch (Exception ex)
        {
            // If the file doesn't exist, return -1
            // Otherwise rethrow the error
            if (ex.Message.Contains("File unavailable")) return -1;
            throw;
        }                           
}