diff --git a/source/eventcore/internal/dlist.d b/source/eventcore/internal/dlist.d
index 5186fc8..8358e5d 100644
--- a/source/eventcore/internal/dlist.d
+++ b/source/eventcore/internal/dlist.d
@@ -59,6 +59,7 @@ struct StackDList(T) {
 
 	void insertAfter(T* item, T* after)
 	{
+		assert(!item.prev && !item.next);
 		if (!after) insertBack(item);
 		else {
 			item.prev = after;