I've seen this on a few sites lately, I'm using 4.11.10

I don't have anything particularly complex on my site, cultiv contact form is about the only package I'm using.

If I try to type in a url to a page that doesn't exist, I get this asp error. Any ideas please? I've searched and turned up almost nothing - one post mentioned a sitemap plugin but I'm not using that plugin.

StartIndex cannot be less than zero.
Parameter name: startIndex

[ArgumentOutOfRangeException: StartIndex cannot be less than zero.
Parameter name: startIndex]
   System.String.InternalSubStringWithChecks(Int32 startIndex, Int32 length, Boolean fAlwaysCopy) +10698829
   System.String.Substring(Int32 startIndex) +12
   AutoFolders.NotFoundHandler.Execute(String url) +538
   Umbraco.Web.Routing.LookupByNotFoundHandlers.HandlePageNotFound(PublishedContentRequest docRequest) +1148
   Umbraco.Web.Routing.LookupByNotFoundHandlers.TrySetDocument(PublishedContentRequest docRequest) +10
   Umbraco.Web.Routing.PublishedContentRequestBuilder.<LookupDocument1>b__1e(IPublishedContentLookup lookup) +15
   System.Linq.Enumerable.Any(IEnumerable`1 source, Func`2 predicate) +146
   Umbraco.Web.Routing.PublishedContentRequestBuilder.LookupDocument1() +198
   Umbraco.Web.Routing.PublishedContentRequestBuilder.LookupDocument() +195
   Umbraco.Web.Routing.PublishedContentRequest.ProcessRequest(HttpContextBase httpContext, UmbracoContext umbracoContext, Action`1 onSuccess) +139
   Umbraco.Web.UmbracoModule.ProcessRequest(HttpContextBase httpContext) +339
   Umbraco.Web.UmbracoModule.<Init>b__7(Object sender, EventArgs e) +80
   System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +136
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +69