Default Value For A Stored Procedure And Conversion Error

When I call this function and the database field 'Login' is null, then I get an error message "Conversion from type 'DBNull' to type 'String' is not valid." I've supplied a default value in my stored procedure and I've also provided a default value in the result value in the function.  How do I get around this?  Thanks 


lblLastUserLogin.Text = GetLastUserLogin().ToString()


Private Function GetLastUserLogin() As String
        Dim result As String = ""
        Dim con As New SqlConnection("server=x.x.x.x;database=database;uid=x;password=x")
        Dim cmd As New SqlCommand("GetLastUserLogin", con)
        cmd.CommandType = CommandType.StoredProcedure
        cmd.Parameters.Add("@UserID", SqlDbType.Int).Value = lblUserID.Text
        cmd.Parameters.Add("@ReturnVal", SqlDbType.SmallDateTime).Direction = ParameterDirection.Output
        Using con
            con.Open()
            cmd.ExecuteNonQuery()
            result = CType(cmd.Parameters("@ReturnVal").Value, String)
        End Using
        Return result
End Function 


CREATE PROCEDURE [dbo].[GetLastUserLogin]
    -- OUTPUT parameter to hold the count.
    @UserID int,
    @ReturnVal smalldatetime = ' ' OUTPUT
AS
    -- This will return the last date returned by the SELECT query.
    Set @ReturnVal = (SELECT MAX(Login) FROM TUserLogs WHERE UserID=@UserID)