Value does not fall within the expected range - CSOM

What is the reason for the below exception message in CSOM code which I received when querying for data from SharePoint Online?

Value does not fall within the expected range.

Code sample:

public static void siteCountHelper(String siteUrl, CredentialSharePoint c, List<SiteCountHelper> results)
     {
         try
         {
             ClientContext client = new ClientContext(siteUrl);
             client = sharepointOnline(client, c);//setting credential
             var web = client.Web;
             client.Load(web, w => w.Webs, w => w.ServerRelativeUrl);
             var lists = client.LoadQuery(client.Web.Lists.Where(temp => temp.BaseType != BaseType.DocumentLibrary));
             var documentLibraries = client.LoadQuery(client.Web.Lists.Where(temp => temp.BaseType == BaseType.DocumentLibrary));
             client.ExecuteQuery();
             SiteCountHelper helper = new SiteCountHelper
             {
                 websCount = web.Webs.Count,
                 listsCount = lists.Count(),
                 documentLibrariesCount = documentLibraries.Count()
             };
             results.Add(helper);
             if (web.Webs.Count != 0)
             {
                 String tempUrl = siteUrl.Replace(web.ServerRelativeUrl, "");
                 foreach (var w in web.Webs)
                 {
                     String mainUrl = tempUrl + w.ServerRelativeUrl;
                     siteCountHelper(mainUrl, c, results); //recursive call
                 }
             }
         }
         catch (Exception e)
         {
             log(e.Message);
         }
     }