diff --git a/stingray/simulator/simulator.py b/stingray/simulator/simulator.py
index ac171738f..e91829a6d 100644
--- a/stingray/simulator/simulator.py
+++ b/stingray/simulator/simulator.py
@@ -8,6 +8,7 @@
 from stingray import utils
 from stingray import Lightcurve
 from stingray import AveragedPowerspectrum
+import math
 
 __all__ = ["Simulator"]
 
@@ -275,10 +276,10 @@ def simple_ir(self, start=0, width=1000, intensity=1):
         """
 
         # Fill in 0 entries until the start time
-        h_zeros = np.zeros(int(start / self.dt))
+        h_zeros = np.zeros(math.ceil(start / self.dt))
 
         # Define constant impulse response
-        h_ones = np.ones(int(width / self.dt)) * intensity
+        h_ones = np.ones(math.ceil(width / self.dt)) * intensity
 
         return np.append(h_zeros, h_ones)