diff --git a/python/swiftest/swiftest/simulation_class.py b/python/swiftest/swiftest/simulation_class.py
index 9fdb9e8ce..bd10b9d06 100644
--- a/python/swiftest/swiftest/simulation_class.py
+++ b/python/swiftest/swiftest/simulation_class.py
@@ -1753,11 +1753,11 @@ def set_unit_system(self,
         TU2S_old = None
 
         if "MU_name" not in dir(self):
-            self.MU_name = None
+            self.MU_name = "MU"
         if "DU_name" not in dir(self):
-            self.DU_name = None
+            self.DU_name = "DU"
         if "TU_name" not in dir(self):
-            self.TU_name = None
+            self.TU_name = "TU"
 
         update_list = []
         if MU is not None or MU2KG is not None: