diff --git a/src/endpoints/bettwaage.py b/src/endpoints/bettwaage.py
index 51a1b60..db61e12 100644
--- a/src/endpoints/bettwaage.py
+++ b/src/endpoints/bettwaage.py
@@ -10,7 +10,7 @@ router = APIRouter()
 
 file_path = "bettwaage.csv"
 
-latest_values = None
+latest_values = []
 zero_values = [0, 0, 0, 0]
 scale_values = [1, 1, 1, 1]
 
@@ -91,7 +91,7 @@ async def delete_file():
 
 @router.post("/zero", tags=["calibration"])
 async def set_zero():
-    if latest_values is None:
+    if not latest_values:
         return HTMLResponse(
             status_code=400, content="Requiring data before setting zeros."
         )