Retrieve the Quote Detail with c#

I'm trying to create a custom workflow (for Dynamics CRM 2011) which must send an email with information on the Details Quote from a quote.

I create it in Visual Studio 2010 with the sdk.

The workflow is triggered manualy from a quote. I am able to retrieve the value of the customerid, but I am unable to get the attached documents or the quotedetails of the Quote, when I launched the workflow I have this exception :

System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.

  at System.Collections.Generic.Dictionary`2.get_Item(TKey key)

  at Microsoft.Xrm.Sdk.Entity.get_Item(String attributeName)

  at CPageCRM.Workflow.QuoteSendMailNotificationRIP.Execute(CodeActivityContext executionContext)

My code is :

//to get the current Quote

Entity preImageEntity = context.PreEntityImages.Values.FirstOrDefault();

//preImageEntity is a Quote because I trigger the workflow from a Quote

//the next two lines work, I can retrieve the good value of the Quote

string natureDevis = Utils.GetOptionSetValueLabel(service, preImageEntity, "new_nature", (OptionSetValue)preImageEntity["new_nature"]);

string prospectDevis = ((EntityReference)preImageEntity["customerid"]).Name;

//I get the exception after that :

List<QuoteDetail> listQuoteDetail = new List<QuoteDetail>();

listQuoteDetail = preImageEntity["quote_details"] as List<QuoteDetail>; //I get the exception

I don't understand why the quote_details doesn't exist in the dictionnary, because when I do :

  Quote devis = new Quote();

  devis.quote_details //<= (the autocompletion is working)

I have the same problem when I try to get sharepointdocumentlocation

Anyone have an explication? How can I retrieve the Quote Details and the document attached to my Quote from the code?

Thanks