diff --git a/python/cosmotool/simu.py b/python/cosmotool/simu.py
index 8be30e9..ff6451c 100644
--- a/python/cosmotool/simu.py
+++ b/python/cosmotool/simu.py
@@ -23,16 +23,16 @@ class SimulationBare(PySimulationBase):
   def merge(self, other):
 
     def _safe_merge(a, b):
-      if b:
-        if a:
+      if b is not None:
+        if a is not None:
           a = [np.append(q, r) for q,r in zip(a,b)]
         else:
           a = b
       return a
       
     def _safe_merge0(a, b):
-      if b:
-        if a:
+      if b is not None:
+        if a is not None:
           a = np.append(a, b)
         else:
           a = b