I have a class called claimuser that has a collection called 'organizations'. I want to query for all the claimusers who have one of the organizations as the value I am querying for. I am getting this strange error 'Key cannot be null'. Please help

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Key cannot be null.
Parameter name: key
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.ArgumentNullException: Key cannot be null.
Parameter name: key

Source Error:

.
.
.

Line 983: if (tstOrg != null)
Line 984: {
Line 985: criteria.CreateCriteria("organizations")
Line 986: .Add(Expression.Eq("Id", tstOrg.Id));
Line 987: }


Source File: C:\Projects\CID\IPATH\BusinessLayer\CID.IPath.BusinessLayer\claimuser.cs Line: 985

Stack Trace:


[ArgumentNullException: Key cannot be null.
Parameter name: key]
System.Collections.Hashtable.get_Item(Object key) +2842117
NHibernate.Impl.SessionFactoryImpl.GetPersister(Type theClass) +24
NHibernate.Impl.CriteriaImpl.GetClassForPath(String rootAlias, String associationPath) +60
NHibernate.Impl.CriteriaImpl.CreateAlias(String rootAlias, String associationPath, String alias) +249
NHibernate.Impl.CriteriaImpl.CreateCriteriaAt(String rootAlias, String associationPath, String alias) +74
NHibernate.Impl.CriteriaImpl.CreateCriteria(String associationPath) +29
CID.IPath.NewBusinessLayer.claimuser.Lookup(ClaimUserSearch cs) in C:\Projects\CID\IPATH\BusinessLayer\CID.IPath.BusinessLayer\claimuser.cs:985
CID.IPath.NewBusinessLayer.claimuser.GetAllExaminers(organization organization, orglocation orglocation, project project) in C:\Projects\CID\IPATH\BusinessLayer\CID.IPath.BusinessLayer\claimuser.cs:1044
claim_ClaimWizard.PopulateProjectDependentDropdowns() in c:\inetpub\wwwroot\CID_IPATH\claim\ClaimWizard.aspx.vb:1533
claim_ClaimWizard.rblReviewProject_SelectedIndexChanged(Object sender, EventArgs e) in c:\inetpub\wwwroot\CID_IPATH\claim\ClaimWizard.aspx.vb:2139
System.Web.UI.WebControls.ListControl.OnSelectedIndexChanged(EventArgs e) +105
System.Web.UI.WebControls.RadioButtonList.RaisePostDataChangedEvent() +145
System.Web.UI.WebControls.RadioButtonList.System.Web.UI.IPostBackDataHandler.RaisePostDataChangedEvent() +7
System.Web.UI.Page.RaiseChangedEvents() +137
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +4778