Index: Ringtoets/Piping/src/Ringtoets.Piping.Forms/TypeConverters/DesignVariableTypeConverter.cs
===================================================================
diff -u -r0f6f1fc37266c0c6cf2c4b6301796ff168d9b955 -r13ea5cffd8f217112d0260ef5fd2c2dbca9d42d7
--- Ringtoets/Piping/src/Ringtoets.Piping.Forms/TypeConverters/DesignVariableTypeConverter.cs	(.../DesignVariableTypeConverter.cs)	(revision 0f6f1fc37266c0c6cf2c4b6301796ff168d9b955)
+++ Ringtoets/Piping/src/Ringtoets.Piping.Forms/TypeConverters/DesignVariableTypeConverter.cs	(.../DesignVariableTypeConverter.cs)	(revision 13ea5cffd8f217112d0260ef5fd2c2dbca9d42d7)
@@ -141,10 +141,13 @@
 
         private static object ContainingObjectFromContext(ITypeDescriptorContext context)
         {
-            if (context != null && context.Instance is DynamicPropertyBag)
+            if (context != null)
             {
-                var bag = (DynamicPropertyBag)context.Instance;
-                return bag.WrappedObject;
+                var bag = context.Instance as DynamicPropertyBag;
+                if (bag != null)
+                {
+                    return bag.WrappedObject;
+                }
             }
             return null;
         }