diff --git a/filter/trie.c b/filter/trie.c
index 8af9015e..fba395d1 100644
--- a/filter/trie.c
+++ b/filter/trie.c
@@ -297,7 +297,7 @@ trie_format(struct f_trie *t, buffer *buf)
   buffer_puts(buf, "[");
 
   if (t->zero)
-    buffer_print(buf, "%I/%d", IPA_NONE, 0);
+    buffer_print(buf, "%I/%d, ", IPA_NONE, 0);
   trie_node_format(t->root, buf);
 
   /* Undo last separator */