diff --git a/gprMax/cmds_singleuse.py b/gprMax/cmds_singleuse.py
index cf562e41..7bcb6aa7 100644
--- a/gprMax/cmds_singleuse.py
+++ b/gprMax/cmds_singleuse.py
@@ -188,7 +188,7 @@ class TimeStepStabilityFactor(UserObjectSingle):
         super().__init__(**kwargs)
         self.order = 4
 
-    def build(self, G, uip):
+    def build(self, model, uip):
         try:
             f = self.kwargs["f"]
         except KeyError:
@@ -202,6 +202,7 @@ class TimeStepStabilityFactor(UserObjectSingle):
             )
             raise ValueError
 
+        G = model.G
         G.dt_mod = f
         G.dt = G.dt * G.dt_mod
 
@@ -220,7 +221,7 @@ class TimeWindow(UserObjectSingle):
         super().__init__(**kwargs)
         self.order = 5
 
-    def build(self, G, uip):
+    def build(self, model, uip):
         # If number of iterations given
         # The +/- 1 used in calculating the number of iterations is to account for
         # the fact that the solver (iterations) loop runs from 0 to < G.iterations
diff --git a/gprMax/grid/fdtd_grid.py b/gprMax/grid/fdtd_grid.py
index f0ee1ea8..60921fe1 100644
--- a/gprMax/grid/fdtd_grid.py
+++ b/gprMax/grid/fdtd_grid.py
@@ -61,7 +61,8 @@ class FDTDGrid:
         self.dz = 0.0
         self.dl: np.ndarray
         self.dt = 0.0
-        self.dt_mod = None  # Time step stability factor
+
+        self.dt_mod = 1.0  # Time step stability factor
         self.iteration = 0  # Current iteration number
         self.iterations = 0  # Total number of iterations
         self.timewindow = 0