500 error on Sitefinity/Public/ResponsiveDesign/layout_trans

After upgrading to 5.1 we're seeing an error on calls to

Sitefinity/Public/ResponsiveDesign/layout_transformations.css

but only when NOT logged in.

Here's the error log text:

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Timestamp: 9/20/2012 7:24:11 PM

Message: HandlingInstanceID: 79072024-08ee-40dd-a429-6e4f4ae440b9
An exception of type 'System.NullReferenceException' occurred and was caught.
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09/20/2012 14:24:11
Type : System.NullReferenceException, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
Message : Object reference not set to an instance of an object.
Source : Telerik.Sitefinity
Help link :
Data : System.Collections.ListDictionaryInternal
TargetSite : Void InitializeControls(Telerik.Sitefinity.Web.UI.GenericContainer)
HResult : -2147467261
Stack Trace :    at Telerik.Sitefinity.Services.Web.UI.PerformanceOptimization.InitializeControls(GenericContainer container)
   at Telerik.Sitefinity.Web.UI.SimpleView.CreateChildControls()
   at System.Web.UI.Control.EnsureChildControls()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Additional Info:

MachineName : REMOVED
TimeStamp : 9/20/2012 7:24:11 PM
FullName : Telerik.Sitefinity.Utilities, Version=5.1.3450.0, Culture=neutral, PublicKeyToken=b28c218413bdf563
AppDomainName : /LM/W3SVC/1/ROOT/Pages-3-129926426198841951
ThreadIdentity : Anonymous
WindowsIdentity : iusr
Requested URL : dev/.../layout_transformations.css


Category: ErrorLog

Priority: 0

EventId: 90000

Severity: Error

Title:Enterprise Library Exception Handling

Machine: REMOVED

App Domain: /LM/W3SVC/1/ROOT/Pages-3-129926426198841951

ProcessId: 3360

Process Name: C:\Windows\SysWOW64\inetsrv\w3wp.exe

Thread Name:

Win32 ThreadId:1256

Extended Properties:
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