diff --git a/filter/tree.c b/filter/tree.c
index 328c7184..1196e630 100644
--- a/filter/tree.c
+++ b/filter/tree.c
@@ -165,6 +165,9 @@ tree_format(struct f_tree *t, buffer *buf)
  
   tree_node_format(t, buf);
 
+  if (buf->pos == buf->end)
+    return;
+
   /* Undo last separator */
   if (buf->pos[-1] != '[')
     buf->pos -= 2;
diff --git a/filter/trie.c b/filter/trie.c
index fba395d1..565ae82f 100644
--- a/filter/trie.c
+++ b/filter/trie.c
@@ -300,6 +300,9 @@ trie_format(struct f_trie *t, buffer *buf)
     buffer_print(buf, "%I/%d, ", IPA_NONE, 0);
   trie_node_format(t->root, buf);
 
+  if (buf->pos == buf->end)
+    return;
+
   /* Undo last separator */
   if (buf->pos[-1] != '[')
     buf->pos -= 2;