don't allow {{c0::

anki/models.py

@@ -533,6 +533,8 @@ select id from notes where mid = ?)""" % " ".join(map),
             ord = map[fname][0]
             ords.update([int(m)-1 for m in re.findall(
                 "{{c(\d+)::[^}]*?}}", sflds[ord])])
+        if -1 in ords:
+            ords.remove(-1)
         return list(ords)
 
     # Sync handling

tests/test_models.py

@@ -157,6 +157,10 @@ def test_cloze():
     f['Text'] = "{{c2::hello}} {{c1::foo}}"
     f.flush()
     assert d.cardCount() == cnt + 1
+    # 0 or negative indices are not supported
+    f['Text'] += "{{c0::zero}} {{c-1:foo}}"
+    f.flush()
+    assert len(f.cards()) == 2
 
 def test_modelChange():
     deck = getEmptyDeck()