Error extending SharePoint 2010 Web Application

When trying to extend an existing SharePoint 2010 web application, I get an error. The Web App is running Claims Authentication. I disabled NTLM Auth on the Default Zone - we want the public (default) zone to be Claims / FBA only. We need to expand the web app to make a new zone to enable Search (requires NTLM Auth).

ERROR: System.FormatException: Input string was not in a correct format. at System.Number.StringToNumber (String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal) at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info) at Microsoft.SharePoint.Administration.SPIisWebSite.Lookup(Uri uri, Boolean requireHighMatchLevel, Int32& instanceId) at Microsoft.SharePoint.Administration.SPProvisioningAssistant.StopSitesWithMatchingBindings(SPIisSettings settings) at Microsoft.SharePoint.Administration.SPProvisioningAssistant.ProvisionWebApplication(String webApplicationName, String applicationPoolName, SPIisSettings[] settings, Boolean adminWebApp) at Microsoft.SharePoint.Administration.SPWebApplication.ProvisionIisWebSi... 8da4ec82-1802-48c3-b595-21a968db9718 06/29/2010 13:24:27.32* w3wp.exe (0x103C) 0x1364 SharePoint Foundation Runtime tkau Unexpected ...tesAsAdministrator() at Microsoft.SharePoint.Administration.SPWebApplication.ProvisionIisWebSites() at Microsoft.SharePoint.Administration.SPWebApplication.Provision() at Microsoft.SharePoint.ApplicationPages.ExtendWebFarmPage.BtnSubmit_Click(Object sender, EventArgs e) at System.Web.UI.WebControls.Button.OnClick(EventArgs e) at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) 8da4ec82-1802-48c3-b595-21a968db9718