InvalidOperationException thrown

I'm having issues with the following code, which is throwing an Invalid Operation Exception

             if (nodeToAdd.nodeCoord != AStarSets.closedSet.First(a => a.nodeCoord == nodeToAdd.nodeCoord).nodeCoord)
             {
                 if (nodeToAdd.nodeCoord == AStarSets.openSet.First(a => a.nodeCoord == nodeToAdd.nodeCoord).nodeCoord)
                 {
                     if (currentHex.GetComponent<HexObject>().hexObjectValuesOut.gValue > Math.Abs(((int)nodeToAdd.nodeCoord.x - x) + ((int)nodeToAdd.nodeCoord.y - y)))
                     {
                         getGCost(nodeToAdd.nodeCoord);
                         currentHex.GetComponent<HexObject>().hexObjectValuesOut.setFValue();
                         nodeToAdd.nodeCost = currentHex.GetComponent<HexObject>().hexObjectValuesOut.fValue;
                         nodeToAdd.parentNode = parentNode;
 
                         AStarSets.openSet.Add(nodeToAdd);
                     }
                 }
             }

This code throws the following exception:

 InvalidOperationException: Operation is not valid due to the current state of the object
 System.Linq.Enumerable.First[AStarNodes] (IEnumerable`1 source, System.Func`2 predicate, Fallback fallback)
 System.Linq.Enumerable.First[AStarNodes] (IEnumerable`1 source, System.Func`2 predicate)
 InfantryPathfinding.addNode (.AStarNodes nodeToAdd, .AStarNodes parentNode, Int32 x, Int32 y, UnityEngine.GameObject currentHex) (at Assets/Hex Grid/Test/InfantryPathfinding.cs:220)
 InfantryPathfinding+<getAboveAndBelowHexes>c__Iterator4.MoveNext () (at Assets/Hex Grid/Test/InfantryPathfinding.cs:210)
 UnityEngine.SetupCoroutine.InvokeMoveNext (IEnumerator enumerator, IntPtr returnValueAddress) (at C:/buildslave/unity/build/Runtime/Export/Coroutines.cs:17)

Any help will be much appreciated