diff --git a/fluidsim/solvers/ns2d/strat/time_stepping.py b/fluidsim/solvers/ns2d/strat/time_stepping.py
index 569c19bde2caf0371a1dba59968a36ac6c08e9d1_Zmx1aWRzaW0vc29sdmVycy9uczJkL3N0cmF0L3RpbWVfc3RlcHBpbmcucHk=..e6653ef320349320e4422161eb970d036b8faec5_Zmx1aWRzaW0vc29sdmVycy9uczJkL3N0cmF0L3RpbWVfc3RlcHBpbmcucHk= 100644
--- a/fluidsim/solvers/ns2d/strat/time_stepping.py
+++ b/fluidsim/solvers/ns2d/strat/time_stepping.py
@@ -166,12 +166,19 @@
         # )
 
         # Removed phase velocity (considered not relevant)
-        maybe_new_dt = min(
-            deltat_CFL,
-            self.deltat_dispersion_relation,
-            self.deltat_group_vel,
-            self.deltat_max,
-        )
+        if not self.coef_group:
+            maybe_new_dt = min(
+                deltat_CFL,
+                self.deltat_dispersion_relation,
+                self.deltat_max,
+            )
+        else:
+            maybe_new_dt = min(
+                deltat_CFL,
+                self.deltat_dispersion_relation,
+                self.deltat_group_vel,
+                self.deltat_max,
+            )
 
 
         if self.params.forcing.enable: