diff --git a/kirppu/forms.py b/kirppu/forms.py
index 2ad1e43..625d074 100644
--- a/kirppu/forms.py
+++ b/kirppu/forms.py
@@ -316,7 +316,8 @@ def clean_receipt(self):
     def clean_code(self):
         data = self.cleaned_data["code"]
         if box_match := re.match(r"box[_ -]?(\d+)$", data):
-            if not Box.objects.filter(pk=int(box_match[1])).exists():
+            # Indefinite check: there may be multiple Box objects with same box_number, across different Events.
+            if not Box.objects.filter(box_number=int(box_match[1])).exists():
                 raise forms.ValidationError("Box {} not found".format(box_match[1]))
             return data
         if not Item.is_item_barcode(data):