diff --git a/graphviz2drawio/mx/Curve.py b/graphviz2drawio/mx/Curve.py
index d8b3352..cc8ec04 100644
--- a/graphviz2drawio/mx/Curve.py
+++ b/graphviz2drawio/mx/Curve.py
@@ -4,8 +4,10 @@
 
 
 class Curve:
-    def __init__(self, start, end, cb, cbset=[]):
+    def __init__(self, start, end, cb, cbset=None):
         """Takes complex numbers for start, end, and list of 4 Bezier control points"""
+        if cbset is None:
+            cbset = []
         self.start = start
         self.end = end
         assert cb is None or len(cb) == 4  # TODO: emit value error instead