In Load Test - "The cast to value type 'System.Int32' failed because the materialized value is null" - entity-framework

Question

when we are debugging below code with single user it is working fine. But while we tried to run Load test when user count goes more than 30 we are getting
"The cast to value type 'System.Int32' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type." exception.
This is code snippet where we are getting exception.
profile.AccessRights = (
from t in entities.Opportunities
join x in entities.OpportunityWorkloadGroups on t.OpportunityId equals x.OpportunityId into oppworkloads
from ow in oppworkloads.DefaultIfEmpty()
join m in entities.OpportunityWorkloads on ow.OpportunityWorkloadGroupId equals m.OpportunityWorkloadGroupId into oppworkloadmodules
from om in oppworkloadmodules.DefaultIfEmpty()
join y in entities.OpportunityUsers on om.OpportunityWorkloadId equals y.OpportunityWorkloadId into users
from ou in users.DefaultIfEmpty()
where (((t.LookupOpportunityStatu.Name.Equals(Constants.EstimationStatusCompletedName) || t.LookupOpportunityStatu.Name.Equals(Constants.OpportunityStatusDiscardedName)
|| t.LookupOpportunityStatu.Name.Equals(OSEConstants.OpportunityStatusCompletedWithDiscards)) && !t.IsRestricted) ? true : ou.User.Alias == null ? true : ou.User.Alias == alias)
select new Microsoft.OneEstimator.DataAccess.Entities.AccessRights()
{
MasterUserId = userId,
MasterUserRole = roleName,
OpportunityId = t.OpportunityId,
IsOpportunityOwner = t.OpportunityOwner == userId ? true : false,
IsRestricted = t.IsRestricted,
StatusName = t.LookupOpportunityStatu.Name,
OpportunityWorkloadId = ow.OpportunityWorkloads.Any(m => m.OpportunityWorkloadGroupId == ow.OpportunityWorkloadGroupId) ? ow.OpportunityWorkloads.FirstOrDefault(m => m.OpportunityWorkloadGroupId == ow.OpportunityWorkloadGroupId).OpportunityWorkloadId : 0,
EstimationId = ow.OpportunityWorkloadGroupId == null ? 0 : ow.IsActive == false ? 0 : ow.OpportunityWorkloadGroupId,
RoleName = ou.Role == null ? string.Empty : ou.User.Alias == alias ? ou.Role.RoleName : string.Empty,
HasRightsForWA = ow.WorkloadGroupVersion != null ? entities.WorkloadGroupUsers.Any(y => y.WorkloadGroupId == ow.WorkloadGroupVersion.WorkloadGroup.WorkloadGroupId && y.UserId == userId && y.Role.RoleName.Equals(OSEConstants.Estimator)) : true,
WorkstreamUserRoleName = ow.WorkloadGroupVersion != null ? entities.WorkloadGroupUsers.FirstOrDefault(y => y.WorkloadGroupId == ow.WorkloadGroupVersion.WorkloadGroup.WorkloadGroupId && y.UserId == userId).Role.RoleName : string.Empty
}).ToList();
I guess I'm doing something wrong with DefaultIfNull() not sure.