App.Config in UnitTesting

I am loading an App.Config section in UnitTest but i got an error

An error occurred creating the configuration section handler for baseConfiguration: Could not load type 'NameSpace.Configurations.BaseConfiguration' from assembly 'System.Configuration, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.

my question is why it is looking at the System.Configuration but supposed to be it will be looking at the assembly that i referenced

i register it this way

 <configSections>
    <section name="baseConfiguration"
             type="NameSpace.Configurations.BaseConfiguration" />
 </configSections>


 <baseConfiguration>
    <clientPortalCookie name="TestCookieName" expirationInMinutes="10" />
  </baseConfiguration>

and my class looks like this

public class BaseConfiguration : ConfigurationSection
    {
        private static BaseConfiguration _current;

        public static BaseConfiguration GetCurrent()
        {
            return _current ?? (_current = (BaseConfiguration) ConfigurationManager.GetSection("baseConfiguration"));
        }  


        [ConfigurationProperty("clientPortalCookie")]
        public ConfClientPortalCookie ClientPortalCookie
        {
            get { return (ConfClientPortalCookie) this["clientPortalCookie"]; }
            set { this["clientPortalCookie"] = value; }
        }
    }

any help would be appreciated

TIA