Cannot Use ConfigurationManager inside Unit Test Project

I'm trying to write a unit test for my project, but it will not let me use the Configuration Manager. Right now my project is set up like

ASP.Net application (all aspx pages)

ProjectCore (all C# files - model)

ProjectTest (all tests)

in my ProjectCore, I am able to access the ConfigurationManager object from System.Configuration and pass information onto the project. However, when I ran a test where the ConfigurationManager is involved, I get the error

System.NullReferenceException: Object reference not set to an instance of an object.

Here is an example of the test

using System.Configuration;

[TestMethod]
public void TestDatabaseExists()
{
    //Error when I declare ConfigurationManager
    Assert.IsNotNull(ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString
}

in my other tests, ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString is what I set my data adapter's configuration string to, and returns a null error on the tests but not when I actually use the website. Any ideas?