diff --git a/src/simudator/core/module.py b/src/simudator/core/module.py
index c28dab000729b49c20f44bde1e8a60fb5a554ee2..66d73fa4ab445dccb0a7d07cb51cb54034fdd278 100644
--- a/src/simudator/core/module.py
+++ b/src/simudator/core/module.py
@@ -81,12 +81,15 @@ class Module:
         """
         pass
 
-    def get_longest_line_len(self, ignore_keys=[]) -> int:
+    def get_longest_line_len(self, ignore_keys=None) -> int:
         """
         Helper function for pretty_print that returns the length of
         the longest line to print for a module.
         """
 
+        if ignore_keys == None:
+            ignore_keys = []
+
         longest_line_len = 0
         state = self.get_state()