"Purchase Apps" fails to connect - generates invalid URL ??

I am having trouble getting the SharePoint 2013 Apps Store to work.  I have single-computer (including SQL) farm  set up up on a Windows Server 2012 VM.  I have created the Apps management and Subscriptions settings service applications, started their service instances, created the app DNS zone and wildcard entry, and configured the App URLs in Central Administration. 

When I try to "Purchase Apps" I get "Sorry, we can't seem to connect to the SharePoint Store..."

Looking the the logs, I find the following: Request to office.com with the following URL.:?lc=en%2DUS&type=3&build=15%2E0%2E4420(I cut out a long string here...) caused a URL exception. Maybe marketplace settings are not setup.

That URL certainly looks invalid.  But  I don't know what "marketplace settings" would be.  The complete ULS log entries are below.  Any ideas?

11/26/2012 09:42:36.75 	w3wp.exe (0x2D20)                       	0x2178	SharePoint Foundation         	App Marketplace               	ahnhh	Medium  	Sending Request to Office.com with the following URL:?lc=en%2DUS&type=3&build=15%2E0%2E4420%2E1017&av=OSU150&origin=EC101785291&corr=%7Bd1ffe49b%2D599b%2Db03e%2Dfcfe%2D6e92d3294cf1%7D	d1ffe49b-599b-b03e-fcfe-6e92d3294cf1
11/26/2012 09:42:36.75 	w3wp.exe (0x2D20)                       	0x2178	SharePoint Foundation         	App Marketplace               	ai234	Unexpected	Request to office.com with the following URL:?lc=en%2DUS&type=3&build=15%2E0%2E4420%2E1017&av=OSU150&origin=EC101785291&corr=%7Bd1ffe49b%2D599b%2Db03e%2Dfcfe%2D6e92d3294cf1%7D&clientip=fe80::24ab:e4e:1e0e:b28a%12 caused a URL exception. Maybe marketplace settings are not setup. Exception details: System.UriFormatException: Invalid URI: The URI scheme is not valid.     at System.Uri.CreateThis(String uri, Boolean dontEscape, UriKind uriKind)     at System.Net.WebRequest.Create(String requestUriString)     at Microsoft.SharePoint.Marketplace.OfficeProxy.OfficeProxy.SendWebRequest(String url, Boolean autoRedirect, String userIP, String postBody, String contentType, Encoding encoding, String jobRunIdLogPrefix)	d1ffe49b-599b-b03e-fcfe-6e92d3294cf1
11/26/2012 09:42:36.75 	w3wp.exe (0x2D20)                       	0x2178	SharePoint Foundation         	App Marketplace               	agdhz	Unexpected	Response from office.com was null or its HTTP status code was not 'OK'.	d1ffe49b-599b-b03e-fcfe-6e92d3294cf1
11/26/2012 09:42:36.75 	w3wp.exe (0x2D20)                       	0x2178	SharePoint Foundation         	App Marketplace               	aep7g	Unexpected	An exception was thrown while running task 'GetOfficeMarkets'. Microsoft.SharePoint.SPException: Sorry, we can't seem to connect to the SharePoint Store. Try again in a bit.     at Microsoft.SharePoint.Marketplace.OfficeProxy.OfficeProxy.GetMarkets(String userDefaultBillingMarket, String userDefaultContentMarket, String userIP, List`1& markets, Boolean& validDefault)     at Microsoft.SharePoint.ApplicationPages.StorefrontBase.GetOfficeMarkets(HttpRequest request, List`1& markets, Boolean& validDefaults)     at Microsoft.SharePoint.ApplicationPages.StorefrontBase.TaskGetOfficeMarkets()     at Microsoft.SharePoint.ApplicationPages.StorefrontBase.TryRunTask(TryGetOutput taskMethod)	d1ffe49b-599b-b03e-fcfe-6e92d3294cf1