diff --git a/include/dwarfs/writer/internal/block_manager.h b/include/dwarfs/writer/internal/block_manager.h
index d6a62d6ab..e37fa94f0 100644
--- a/include/dwarfs/writer/internal/block_manager.h
+++ b/include/dwarfs/writer/internal/block_manager.h
@@ -37,7 +37,7 @@ class block_manager {
   size_t get_logical_block() const;
   void set_written_block(size_t logical_block, size_t written_block,
                          fragment_category::value_type category);
-  void map_logical_blocks(std::vector<chunk_type>& vec);
+  void map_logical_blocks(std::vector<chunk_type>& vec) const;
   std::vector<fragment_category::value_type>
   get_written_block_categories() const;
 
diff --git a/src/writer/internal/block_manager.cpp b/src/writer/internal/block_manager.cpp
index 3e48d3200..c275709e3 100644
--- a/src/writer/internal/block_manager.cpp
+++ b/src/writer/internal/block_manager.cpp
@@ -45,12 +45,12 @@ void block_manager::set_written_block(size_t logical_block,
   block_map_[logical_block] = std::make_pair(written_block, category);
 }
 
-void block_manager::map_logical_blocks(std::vector<chunk_type>& vec) {
+void block_manager::map_logical_blocks(std::vector<chunk_type>& vec) const {
   std::lock_guard lock{mx_};
   for (auto& c : vec) {
     size_t block = c.get_block();
     assert(block < num_blocks_);
-    c.block() = block_map_[block].value().first;
+    c.block() = block_map_.at(block).value().first;
   }
 }