Login Error using SHA1 Hashed Password with AspNet.Identity 2.0

I am trying to use AspNet.Identity 2.0 with Database First Approach. I have successfully mapped the required tables to my database and I am able to run my application and register my user. The problem I encounter is when I log off the user and try to login again I get an error message in the browser that reads:

The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters.

The error source seems to be the line below which is always returning failure:

var result = await SignInManager.PasswordSignInAsync(user.UserName, model.Password, model.RememberMe, shouldLockout: false);

I have used the following code to hash my password for storage in the database:

public class MyPasswordHasher : PasswordHasher
{
    public override string HashPassword(string password)
    {
        var textToHash = Encoding.Default.GetBytes(password);
        using (SHA1Managed sha1 = new SHA1Managed())
        {
            var result = sha1.ComputeHash(textToHash);
            var resultText =  Convert.ToBase64String(result);
            resultText = resultText.Replace("+", "_");
            return resultText;
        }
    }

    public override PasswordVerificationResult VerifyHashedPassword(
      string hashedPassword, string providedPassword)
    {
        return hashedPassword.Equals(HashPassword(providedPassword)) ? PasswordVerificationResult.Success : PasswordVerificationResult.Failed;

    }
}

Again, when I use the method below to check the user password at login, it is always returning false:

SignInManager.UserManager.CheckPassword(user, model.Password);

Please help me to eliminate these errors. So far the closest problem I have come across is here: ASP.Net Identity Manual Password Hashing --- and it does not exactly address this error I am having. Thanks.