diff --git a/blond/input_parameters/rf_parameters.py b/blond/input_parameters/rf_parameters.py
index 984645b6ead6ffe790351f0d8635e231937257cf..8de9e8412d200291afdb1c049b58d86e3a833512 100644
--- a/blond/input_parameters/rf_parameters.py
+++ b/blond/input_parameters/rf_parameters.py
@@ -505,14 +505,13 @@ def calculate_phi_s(RFStation, Particle=Proton(),
 
         denergy = np.append(RFStation.delta_E, RFStation.delta_E[-1])
         acceleration_ratio = denergy / (Particle.charge * RFStation.voltage[0, :])
-        acceleration_test = np.where((acceleration_ratio > -1) *
-                                     (acceleration_ratio < 1) is False)[0]
-
+        acceleration_test = ((acceleration_ratio > -1) & (acceleration_ratio < 1)) == 0
+        
         # Validity check on acceleration_ratio
-        if acceleration_test.size > 0:
+        if np.count_nonzero(acceleration_test) > 0:
             print("WARNING in calculate_phi_s(): acceleration is not " +
                   "possible (momentum increment is too big or voltage too " +
-                  "low) at index " + str(acceleration_test))
+                  "low) at index " + str(acceleration_test.nonzero()[0]))
 
         phi_s = np.arcsin(acceleration_ratio)