diff --git a/lib/query.cc b/lib/query.cc
index c155470a..07e695b9 100644
--- a/lib/query.cc
+++ b/lib/query.cc
@@ -260,17 +260,20 @@ _notmuch_mset_messages_move_to_next (notmuch_messages_t *messages)
 static notmuch_bool_t
 _notmuch_doc_id_set_init (void *ctx,
 			  notmuch_doc_id_set_t *doc_ids,
-			  GArray *arr, unsigned int bound)
+			  GArray *arr)
 {
-    size_t count = (bound + sizeof (doc_ids->bitmap[0]) - 1) /
-	sizeof (doc_ids->bitmap[0]);
-    unsigned int *bitmap = talloc_zero_array (ctx, unsigned int, count);
+    unsigned int max = 0;
+    unsigned int *bitmap;
+
+    for (unsigned int i = 0; i < arr->len; i++)
+	max = MAX(max, g_array_index (arr, unsigned int, i));
+    bitmap = talloc_zero_array (ctx, unsigned int, 1 + max / sizeof (*bitmap));
 
     if (bitmap == NULL)
 	return FALSE;
 
     doc_ids->bitmap = bitmap;
-    doc_ids->bound = bound;
+    doc_ids->bound = max + 1;
 
     for (unsigned int i = 0; i < arr->len; i++) {
 	unsigned int doc_id = g_array_index (arr, unsigned int, i);
@@ -315,7 +318,6 @@ notmuch_query_search_threads (notmuch_query_t *query)
 {
     notmuch_threads_t *threads;
     notmuch_messages_t *messages;
-    Xapian::docid max_doc_id = 0;
 
     threads = talloc (query, notmuch_threads_t);
     if (threads == NULL)
@@ -335,7 +337,6 @@ notmuch_query_search_threads (notmuch_query_t *query)
     while (notmuch_messages_valid (messages)) {
 	unsigned int doc_id = _notmuch_mset_messages_get_doc_id (messages);
 	g_array_append_val (threads->doc_ids, doc_id);
-	max_doc_id = MAX (max_doc_id, doc_id);
 	notmuch_messages_move_to_next (messages);
     }
     threads->doc_id_pos = 0;
@@ -343,7 +344,7 @@ notmuch_query_search_threads (notmuch_query_t *query)
     talloc_free (messages);
 
     if (! _notmuch_doc_id_set_init (threads, &threads->match_set,
-				    threads->doc_ids, max_doc_id + 1)) {
+				    threads->doc_ids)) {
 	talloc_free (threads);
 	return NULL;
     }