diff --git a/pyfstat/grid_based_searches.py b/pyfstat/grid_based_searches.py
index 46d0a9ed7b5b4483af88cd73393af7889196d12a..cf6a98262dad94a8a14c3d4e5a53d73f9bfd3ffb 100644
--- a/pyfstat/grid_based_searches.py
+++ b/pyfstat/grid_based_searches.py
@@ -599,7 +599,7 @@ class TransientGridSearch(GridSearch):
             detstat = self.search.get_det_stat(*vals)
             windowRange = getattr(self.search, "windowRange", None)
             FstatMap = getattr(self.search, "FstatMap", None)
-            self.timingFstatMap += getattr(self.search, "timingFstatMap", None)
+            self.timingFstatMap += getattr(self.search, "timingFstatMap", 0.0)
             thisCand = list(vals) + [detstat]
             if getattr(self, "transientWindowType", None):
                 if self.tCWFstatMapVersion == "lal":