diff --git a/src/SimNDT/run_setup/runSimulator.py b/src/SimNDT/run_setup/runSimulator.py
index ee9a7cf..0af799f 100644
--- a/src/SimNDT/run_setup/runSimulator.py
+++ b/src/SimNDT/run_setup/runSimulator.py
@@ -418,8 +418,12 @@ def read_simulation(sim_params):
         
         if "dx" in sim_params["Simulation"]:
           SimNDT_Simulation.preferredDX=sim_params["Simulation"]["dx"]
+        else:
+          SimNDT_Simulation.preferredDX=None
         if "dt" in sim_params["Simulation"]:
           SimNDT_Simulation.preferredDT=sim_params["Simulation"]["dt"]
+        else:
+          SimNDT_Simulation.preferredDT=None
         
         print("Finished reading Simulation section.")
         print(repr(SimNDT_Simulation))