c# - Value cannot be null, r nparameter name source -

i confused. have 2 statements same , 1 works , other receives error - value cannot null. r nparameter name source. read receiving error because null in linq expression. can tell nothing null.

the first if statement works. when person selects 'select' list of burn project list of burnpiles displayed below burnproject list. (this works). when person selects 'select' list of burnpiles list of requestedburns display below it. gives me null error. @ 1 time did work doesn't.

i @ loss of went wrong. know requestedburn table starts @ record #2 shouldn't have it. burnpile records have been using have associated requestedburns.

        //when 'select' chosen list of burn projects list of burn piles          //associated specific burn project display below it.         if (burnerid != null)         {             viewbag.burnerid = burnerid.value;             viewmodel.burnpiles = viewmodel.burnprojects.where(                 b => b.burnerid == burnerid.value).single().burnpiles;         }          //when 'select' chosen list of burn piles list of requested          //burns associated specific burn pile displayed below it.         if (burnpileid != null)         {             viewbag.burnpilesid = burnpileid.value;             viewmodel.requestedburns = viewmodel.burnpiles.where(                 x => x.burnpilesid == burnpileid).single().requestedburns;          } 

viewmodel.burnpiles = viewmodel.burnprojects.where(             b => b.burnerid == burnerid.value).single().burnpiles; 

could setting viewmodel.burnpiles null.

or

viewmodel.burnpiles.where(             x => x.burnpilesid == burnpileid).single() 

is returning nothing when try , access requestedburns throws exception.