ID4257: X.509 certificate 'CN=SharePoint Security Token Service, OU=SharePoint, O=Microsoft, C=US' validation failed by the token handler.

I am trying to build a Security Token, Serialize, Read and Validate it, all inside a Console Application. Following is the code. I am getting an exception while trying to Validate the Token. The SharePoint 2013 site I am using here uses FBA Claims and the site is a Multi Tenant site.

 string siteUrl = "url of the site";
                Uri uri = new Uri(siteUrl);
                System.IdentityModel.Tokens.SecurityToken token = SPSecurityContext.SecurityTokenForFormsAuthentication(uri, "CustomMembershipProvider", "CustomRoleProvider", "user1", "password1", SPFormsAuthenticationOption.PersistentSignInRequest);
                if (token != null)
                {
                    string assertionXML = ((System.IdentityModel.Tokens.GenericXmlSecurityToken)(token)).TokenXml.OwnerDocument.InnerXml;
                    using (StringReader sr = new StringReader(assertionXML))
                    {
                        using (XmlReader reader = XmlReader.Create(sr))
                        {
                            if (!reader.ReadToFollowing("saml:Assertion"))
                            {
                                throw new Exception("Assertion not found!");
                            }
                            SecurityTokenHandlerCollection collection = SecurityTokenHandlerCollection.CreateDefaultSecurityTokenHandlerCollection();
                            collection.ElementAt(2).Configuration.AudienceRestriction.AllowedAudienceUris.Add(uri);

                            System.IdentityModel.Tokens.SecurityToken newToken = collection.ReadToken(reader.ReadSubtree());
                            ReadOnlyCollection<System.Security.Claims.ClaimsIdentity> claims = collection.ValidateToken(newToken); <-- This line throws exception
                        }
                    }
                }

The Inner Exception message is as given below.

"The X.509 certificate CN=SharePoint Security Token Service, OU=SharePoint, O=Microsoft, C=US is not in the trusted people store. 
The X.509 certificate CN=SharePoint Security Token Service, OU=SharePoint, O=Microsoft, C=US chain building failed. 
The certificate that was used has a trust chain that cannot be verified. 
Replace the certificate or change the certificateValidationMode. 
A certificate chain could not be built to a trusted root authority"

Can anyone please help?

Ven