Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.ArgumentNullException: Value cannot be null.
Parameter name: mapping

Source Error: 


Line 45:     #endregion
Line 46:        
Line 47:        public db() : 
Line 48:                base(global::data.Properties.Settings.Default.nanocrmConnectionString, mappingSource)
Line 49:        {

this is what i get if i implement such class:

partial class db
{
    static db _db = new db();

    public static db GetInstance()
    {
        return _db;
    }
}

db is a linq2sql datacontext

why this hapenned and how to solve this?