OAB Generator Assistant Error

Hi

Occasionally I see the  error 178008 OAB Generator Assistance. 

Has anyone see this before and can tell me what the issue might be.

Cheers

Laurie

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Log Name:      Application
Source:        MSExchangeMailboxAssistants
Date:          1/02/2015 8:08:06 PM
Event ID:      17008
Task Category: OAB Generator Assistant
Level:         Error
Description:
OABGeneratorAssistant encountered an error not related to any specific OAB.       
System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.Exchange.MailboxAssistants.Assistants.OABGenerator.OABGeneratorAssistant.GetOABsFromAD(OABGeneratorTaskContext context, Guid oabObjectGuid)
   at Microsoft.Exchange.MailboxAssistants.Assistants.OABGenerator.OABGeneratorAssistant.InitializeOABGeneration(AssistantTaskContext assistantTaskContext)
   at Microsoft.Exchange.MailboxAssistants.Assistants.OABGenerator.OABGeneratorAssistant.<>c__DisplayClasse.<ProcessAssistantStep>b__a()
   at Microsoft.Exchange.Common.IL.ILUtil.DoTryFilterCatch(TryDelegate tryDelegate, FilterDelegate filterDelegate, CatchDelegate catchDelegate)
   at Microsoft.Exchange.Common.IL.ILUtil.DoTryFilterCatch(TryDelegate tryDelegate, FilterDelegate filterDelegate, CatchDelegate catchDelegate)     
Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
  <System>
    <Provider Name="MSExchangeMailboxAssistants" />
    <EventID Qualifiers="49156">17008</EventID>
    <Level>2</Level>
    <Task>17</Task>
    <Keywords>0x80000000000000</Keywords>
    <TimeCreated SystemTime="2015-02-01T10:38:06.000000000Z" />
    <EventRecordID>5032334</EventRecordID>
    <Channel>Application</Channel>
    <Security />
  </System>
  <EventData>
    <Data>System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.Exchange.MailboxAssistants.Assistants.OABGenerator.OABGeneratorAssistant.GetOABsFromAD(OABGeneratorTaskContext context, Guid oabObjectGuid)
   at Microsoft.Exchange.MailboxAssistants.Assistants.OABGenerator.OABGeneratorAssistant.InitializeOABGeneration(AssistantTaskContext assistantTaskContext)
   at Microsoft.Exchange.MailboxAssistants.Assistants.OABGenerator.OABGeneratorAssistant.&lt;&gt;c__DisplayClasse.&lt;ProcessAssistantStep&gt;b__a()
   at Microsoft.Exchange.Common.IL.ILUtil.DoTryFilterCatch(TryDelegate tryDelegate, FilterDelegate filterDelegate, CatchDelegate catchDelegate)
   at Microsoft.Exchange.Common.IL.ILUtil.DoTryFilterCatch(TryDelegate tryDelegate, FilterDelegate filterDelegate, CatchDelegate catchDelegate)</Data>
  </EventData>
</Event>