diff --git a/treelib/tree.py b/treelib/tree.py
index 62d3549..cac0262 100644
--- a/treelib/tree.py
+++ b/treelib/tree.py
@@ -858,9 +858,9 @@ def to_dict(self, nid=None, key=None, sort=True, reverse=False, with_data=False)
                     {ntag: {"data": self[nid].data}}
             return tree_dict
 
-    def to_json(self, with_data=False, sort=True, reverse=False):
+    def to_json(self, with_data=False, sort=True, reverse=False, key=None):
         """To format the tree in JSON format."""
-        return json.dumps(self.to_dict(with_data=with_data, sort=sort, reverse=reverse))
+        return json.dumps(self.to_dict(with_data=with_data, sort=sort, reverse=reverse, key=key))
 
     def to_graphviz(self, filename=None, shape='circle', graph='digraph'):
         """Exports the tree in the dot format of the graphviz software"""