diff --git a/seminar/treelib.py b/seminar/treelib.py
index 2fb53c8e..a17d9aee 100644
--- a/seminar/treelib.py
+++ b/seminar/treelib.py
@@ -229,7 +229,7 @@ def create_node_after(predecessor, type, **kwargs):
 	from seminar.models import TreeNode
 	if predecessor is None:
 		raise TreeLibError("Nelze vyrábět sirotky! (predecessor=None)")
-	if not isinstance(type, TreeNode):
+	if not issubclass(type, TreeNode):
 		raise TreeLibError("Nový node není node!")
 	new_node = type.objects.create(**kwargs)
 	new_node.root = predecessor.root
@@ -246,7 +246,7 @@ def create_child(parent, type, **kwargs):
 	from seminar.models import TreeNode
 	if parent is None:
 		raise TreeLibError("Nelze vyrábět sirotky! (parent=None)")
-	if not isinstance(type, TreeNode):
+	if not issubclass(type, TreeNode):
 		raise TreeLibError("Nový node není node!")
 	new_node = type.objects.create(**kwargs)
 	new_node.root = parent.root
@@ -284,7 +284,7 @@ def create_node_before(successor, type, **kwargs):
 	from seminar.models import TreeNode
 	if successor is None:
 		raise TreeLibError("Nelze vyrábět sirotky! (successor=None)")
-	if not isinstance(type, TreeNode):
+	if not issubclass(type, TreeNode):
 		raise TreeLibError("Nový node není node!")
 	if safe_pred(successor) is not None:
 		# Easy: přidáme za předchůdce