I get this error "input string does not contain a valid encoding of the 'RSA' 'Modulus' parameter"

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
using System.Xml;
using System.Xml.Linq;
using System.Security.Cryptography;

namespace CryptographicClasses
{
    class BigEncryption
    {
        public static void PubToXml()
        {
            var txt = string.Empty;
            using (var stream = File.OpenText("public_key.txt"))
            {
                txt = stream.ReadToEnd();
            }

            var xml = new XDocument(new XElement("Data", new XElement("info", txt)));
            xml.Save(Path.ChangeExtension("public_key.txt", ".xml"));
        }

        public static void PriToXml()
        {
            var txt = string.Empty;
            using (var stream = File.OpenText("private_key.txt"))
            {
                txt = stream.ReadToEnd();
            }

            var xml = new XDocument(new XElement("Data", new XElement("info", txt)));
            xml.Save(Path.ChangeExtension("private_key.txt", ".xml"));
        }

        public static string Encrypt()
        {
            try
            {
                PubToXml();
                RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
                StreamReader readme = File.OpenText("public_key.xml");
                string rsaXml = readme.ReadToEnd();
                readme.Close();
                rsa.FromXmlString(rsaXml);
                Console.WriteLine("Enter text here");
                string message = Console.ReadLine();

                byte[] encrypted = rsa.Encrypt(System.Text.ASCIIEncoding.ASCII.GetBytes(message), false);

                FileStream steam = new FileStream("Lmessage.dat", FileMode.Create);
                steam.Write(encrypted, 0, encrypted.Length);
                steam.Close();
                
            }

            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
            
            return "0";
        }

        public static string Decrypt()
        {
            PriToXml();
            RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
            StreamReader readme = File.OpenText("private_key.xml");
            string rsaXml = readme.ReadToEnd();
            readme.Close();
            rsa.FromXmlString(rsaXml);
            string message = Console.ReadLine();

            byte[] decrypted = rsa.Decrypt(System.Text.ASCIIEncoding.ASCII.GetBytes(message), false);

            FileStream steam = new FileStream("Lmessage.dat", FileMode.Create);
            steam.Write(decrypted, 0, decrypted.Length);
            steam.Close();
            return "0";
        }
    }
}