diff --git a/scripts/add_electricity.py b/scripts/add_electricity.py
index a799caec..06f6348d 100755
--- a/scripts/add_electricity.py
+++ b/scripts/add_electricity.py
@@ -586,7 +586,7 @@ def attach_hydro(n, costs, ppl, profile_hydro, hydro_capacities, carriers, **par
         # fill missing max hours to params value and
         # assume no natural inflow due to lack of data
         max_hours = params.get("PHS_max_hours", 6)
-        phs = phs.replace({"max_hours": {0: max_hours}})
+        phs = phs.replace({"max_hours": {0: max_hours, np.nan: max_hours}})
         n.madd(
             "StorageUnit",
             phs.index,