diff --git a/persistent-data/data-structures/bitset.cc b/persistent-data/data-structures/bitset.cc
index 98805fa..fae1f34 100644
--- a/persistent-data/data-structures/bitset.cc
+++ b/persistent-data/data-structures/bitset.cc
@@ -82,7 +82,7 @@ namespace persistent_data {
 			}
 
 			void walk_bitset(bitset_visitor &v) const {
-				bit_visitor vv(v);
+				bit_visitor vv(v, nr_bits_);
 				damage_visitor dv(v);
 				array_.visit_values(vv, dv);
 			}
@@ -90,18 +90,20 @@ namespace persistent_data {
 		private:
 			class bit_visitor {
 			public:
-				bit_visitor(bitset_visitor &v)
-				: v_(v) {
+				bit_visitor(bitset_visitor &v, unsigned nr_bits)
+					: v_(v),
+					  nr_bits_(nr_bits) {
 				}
 
 				void visit(uint32_t word_index, uint64_t word) {
 					uint32_t bit_index = word_index * 64;
-					for (unsigned bit = 0; bit < 64; bit++, bit_index++)
-						v_.visit(bit_index, !!(word & (1 << bit)));
+					for (unsigned bit = 0; bit < 64 && bit_index < nr_bits_; bit++, bit_index++)
+						v_.visit(bit_index, !!(word & (1ULL << bit)));
 				}
 
 			private:
 				bitset_visitor &v_;
+				unsigned nr_bits_;
 			};
 
 			class damage_visitor {