diff --git a/btlib/src/btlib/bts.py b/btlib/src/btlib/bts.py
index 0f9b10d..9c98e0e 100644
--- a/btlib/src/btlib/bts.py
+++ b/btlib/src/btlib/bts.py
@@ -34,15 +34,20 @@ def _get_siblings(elm):
     return [ch for ch in elm.parent.children if isinstance(ch, Tag)]
 
 
-def _get_unique_id(elm: BeautifulSoup, tree_id: int) -> int:
+def _get_unique_id(elm: BeautifulSoup, tree_id: int) -> str:
+    """
+    Get a unique id for the node.
+    
+    This is a byte string that is unique for the node in the graph. It is used to identify the node
+    in the graph.
+    """
     if elm.name == 'BehaviorTree':
-        # we need a one in at first position, because 00 evaluates to 0
-        return tree_id + 10
-    parents_ch_list = _get_siblings(elm)
-    n_chars_of_nr = floor(log10(len(parents_ch_list)) + 1)
-    return int(
-        str(_get_unique_id(elm.parent, tree_id)) + str(
-            parents_ch_list.index(elm)).zfill(n_chars_of_nr))
+        return f'{tree_id:02x}'
+    # which number sibling is this node
+    sibling_no: int = _get_siblings(elm).index(elm)
+    this_id: str = _get_unique_id(elm.parent, tree_id)
+    this_id += f'{sibling_no:02x}'
+    return this_id
 
 
 def _get_attrs(elm: BeautifulSoup) -> dict:
diff --git a/btlib/test/systemtests/test_bt_to_fsm.py b/btlib/test/systemtests/test_bt_to_fsm.py
index 9072f84..f7a2662 100644
--- a/btlib/test/systemtests/test_bt_to_fsm.py
+++ b/btlib/test/systemtests/test_bt_to_fsm.py
@@ -38,14 +38,14 @@ def test_inverter(self):
 
         # check the existence of the edges
         for port in ['success', 'failure', 'running']:
-            self.assertTrue(fsm.has_edge('1000_ServiceBtCondition', port))
-        self.assertTrue(fsm.has_edge('tick', '1000_ServiceBtCondition'))
+            self.assertTrue(fsm.has_edge('000000_ServiceBtCondition', port))
+        self.assertTrue(fsm.has_edge('tick', '000000_ServiceBtCondition'))
 
         # check the labels of the edges
-        self.assertEqual(fsm.edges['1000_ServiceBtCondition', 'success']['label'], 'on_failure')
-        self.assertEqual(fsm.edges['1000_ServiceBtCondition', 'failure']['label'], 'on_success')
-        self.assertEqual(fsm.edges['1000_ServiceBtCondition', 'running']['label'], 'on_running')
-        self.assertEqual(fsm.edges['tick', '1000_ServiceBtCondition']['label'], 'on_tick')
+        self.assertEqual(fsm.edges['000000_ServiceBtCondition', 'success']['label'], 'on_failure')
+        self.assertEqual(fsm.edges['000000_ServiceBtCondition', 'failure']['label'], 'on_success')
+        self.assertEqual(fsm.edges['000000_ServiceBtCondition', 'running']['label'], 'on_running')
+        self.assertEqual(fsm.edges['tick', '000000_ServiceBtCondition']['label'], 'on_tick')
 
     def test_simple(self):
         """Test that the conversion from a Behavior Tree to a FSM works."""
diff --git a/btlib/test/unittests/test_unittest_bts.py b/btlib/test/unittests/test_unittest_bts.py
index b347f59..5e20051 100644
--- a/btlib/test/unittests/test_unittest_bts.py
+++ b/btlib/test/unittests/test_unittest_bts.py
@@ -63,7 +63,7 @@ def test_xml_to_networkx(self):
         # all nodes are in the graph and the xpi
         self.assertEqual(g.number_of_nodes(), 4)
         self.assertEqual(len(xpi), 4)
-        for x in [10, 100, 1000, 1001]:
+        for x in ['00', '0000', '000000', '000001']:
             self.assertIn(x, g.nodes)
             self.assertIn(x, xpi.keys())