diff --git a/unpack-trees.c b/unpack-trees.c
index ad3e9a04fe..2515be6a60 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -1157,6 +1157,8 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	if (o->dst_index) {
 		discard_index(o->dst_index);
 		*o->dst_index = o->result;
+	} else {
+		discard_index(&o->result);
 	}
 
 done: