diff --git a/api/s4_tanks.py b/api/s4_tanks.py
index 31d53c9..7650520 100644
--- a/api/s4_tanks.py
+++ b/api/s4_tanks.py
@@ -192,6 +192,8 @@ def inp_out_tank(name: str) -> list[str]:
         min_vol = obj['min_vol']
         vol_curve = obj['vol_curve'] if obj['vol_curve'] != None else ''
         overflow = obj['overflow'] if obj['overflow'] != None else ''
+        if vol_curve == '' and overflow != '':
+            vol_curve = '*'
         desc = ';'
         lines.append(f'{id} {elevation} {init_level} {min_level} {max_level} {diameter} {min_vol} {vol_curve} {overflow} {desc}')
     return lines