diff --git a/mocha-dotnet/src/lib/Mocha.Core/Oms.cs b/mocha-dotnet/src/lib/Mocha.Core/Oms.cs
index 9d8f93b..a709dd6 100644
--- a/mocha-dotnet/src/lib/Mocha.Core/Oms.cs
+++ b/mocha-dotnet/src/lib/Mocha.Core/Oms.cs
@@ -402,8 +402,18 @@ public abstract class Oms
 			InstanceHandle ihSourceClass = GetRelatedInstance(relationship, GetInstance(KnownRelationshipGuids.Relationship__has_source__Class));
 			InstanceHandle ihDestinationClass = GetRelatedInstance(relationship, GetInstance(KnownRelationshipGuids.Relationship__has_destination__Class));
 			List<InstanceHandle> ignoreClasses = new List<InstanceHandle>();
-			ignoreClasses.Add(GetInstance(KnownInstanceGuids.Classes.Instance));
-			ignoreClasses.Add(GetInstance(KnownInstanceGuids.Classes.ExecutableReturningWorkData));
+			if (TryGetInstance(KnownInstanceGuids.Classes.Instance, out InstanceHandle __ih_1))
+			{
+				ignoreClasses.Add(__ih_1);
+			}
+			if (TryGetInstance(KnownInstanceGuids.Classes.ExecutableReturningWorkData, out InstanceHandle __ih_2))
+			{
+				ignoreClasses.Add(__ih_2);
+			}
+			if (TryGetInstance(KnownInstanceGuids.Classes.Calculation, out InstanceHandle __ih_3))
+			{
+				ignoreClasses.Add(__ih_3);
+			}
 
 			if (ihSourceClass != InstanceHandle.Empty)
 			{