In Dynamic CRM 2011,Administrator can't disable a businessunit record.

I am a crm system administrator,I want to delete a businessunit record which don't have any child businessunit and any other entity recrods,but I can't delete it .and I got a system errory at below.

Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=5.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: System.Web.HttpUnhandledException: Microsoft Dynamics CRM has experienced an error. Reference number for administrators or support: #DC06CA76Detail: 
<OrganizationServiceFault xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.microsoft.com/xrm/2011/Contracts">
  <ErrorCode>-2147220970</ErrorCode>
  <ErrorDetails xmlns:d2p1="http://schemas.datacontract.org/2004/07/System.Collections.Generic" />
  <Message>System.Web.HttpUnhandledException: Microsoft Dynamics CRM has experienced an error. Reference number for administrators or support: #DC06CA76</Message>
  <Timestamp>2013-04-28T07:42:55.5953928Z</Timestamp>
  <InnerFault>
    <ErrorCode>-2147213984</ErrorCode>
    <ErrorDetails xmlns:d3p1="http://schemas.datacontract.org/2004/07/System.Collections.Generic" />
    <Message>Not disabled business unit cannot be deleted.</Message>
    <Timestamp>2013-04-28T07:42:55.5953928Z</Timestamp>
    <InnerFault i:nil="true" />
    <TraceText i:nil="true" />
  </InnerFault>
  <TraceText i:nil="true" />
</OrganizationServiceFault>

---

windows live Id :hellohongfu@hotmail.com