diff --git a/src/cal_tools/dssclib.py b/src/cal_tools/dssclib.py
index 0147eb3e5116945567bd6e02e09d5b3e7fdf0d53..a17ec3648524c7449e3e20cae44c4348965bbcc8 100644
--- a/src/cal_tools/dssclib.py
+++ b/src/cal_tools/dssclib.py
@@ -16,7 +16,7 @@ def get_num_cells(fname, h5path):
         if cells == []:
             return
         maxcell = np.max(cells)
-        return maxcell
+        return maxcell+1
 
 
 def get_pulseid_checksum(fname, h5path, h5path_idx):