diff --git a/prolefeeder.py b/prolefeeder.py
index cb10917..8236767 100644
--- a/prolefeeder.py
+++ b/prolefeeder.py
@@ -78,6 +78,8 @@ def prepare_download(form):
     for i, source in enumerate(sources):
         if source['start_time'] <= form.start_time.data < source['start_time'] + source['length']:
             start_index = i
+    if start_index is None:
+        return None
     sources = sources[start_index:]
 
     # Seek into the first file