diff --git a/pack-bitmap-write.c b/pack-bitmap-write.c
index c43375bd34..4fcfaed428 100644
--- a/pack-bitmap-write.c
+++ b/pack-bitmap-write.c
@@ -683,7 +683,7 @@ static void write_hash_cache(struct hashfile *f,
 	}
 }
 
-void bitmap_writer_set_checksum(unsigned char *sha1)
+void bitmap_writer_set_checksum(const unsigned char *sha1)
 {
 	hashcpy(writer.pack_checksum, sha1);
 }
diff --git a/pack-bitmap.h b/pack-bitmap.h
index 3d3ddd7734..f3a57ca065 100644
--- a/pack-bitmap.h
+++ b/pack-bitmap.h
@@ -75,7 +75,7 @@ int bitmap_has_oid_in_uninteresting(struct bitmap_index *, const struct object_i
 off_t get_disk_usage_from_bitmap(struct bitmap_index *, struct rev_info *);
 
 void bitmap_writer_show_progress(int show);
-void bitmap_writer_set_checksum(unsigned char *sha1);
+void bitmap_writer_set_checksum(const unsigned char *sha1);
 void bitmap_writer_build_type_index(struct packing_data *to_pack,
 				    struct pack_idx_entry **index,
 				    uint32_t index_nr);