diff --git a/caching/metadata_dump.cc b/caching/metadata_dump.cc
index d0e5367..735d09b 100644
--- a/caching/metadata_dump.cc
+++ b/caching/metadata_dump.cc
@@ -64,7 +64,7 @@ namespace {
 
 	class hint_emitter : public hint_visitor {
 	public:
-		hint_emitter(emitter::ptr e, set<block_address> valid_blocks)
+		hint_emitter(emitter::ptr e, set<block_address> &valid_blocks)
 			: e_(e),
 			  valid_blocks_(valid_blocks) {
 		}