diff --git a/lib/builder_web.ml b/lib/builder_web.ml
index b686262..54ec98b 100644
--- a/lib/builder_web.ml
+++ b/lib/builder_web.ml
@@ -552,15 +552,12 @@ let routes ~datadir ~cachedir ~configdir ~expired_jobs =
   in
 
 
-  let resolve_artifact id_opt conn =
-    let default_file : Builder_db.file = {
-      filepath = Fpath.v "null";
-      sha256 = "0";
-      size = -1;
-    } in
+  let resolve_artifact_size id_opt conn =
     match id_opt with
-    | None -> Lwt.return_ok default_file
-    | Some id -> Model.build_artifact_by_id id conn
+    | None -> Lwt.return_ok None
+    | Some id ->
+      Model.build_artifact_by_id id conn >|= fun file ->
+      Some file.size
 
   in
 
@@ -584,16 +581,16 @@ let routes ~datadir ~cachedir ~configdir ~expired_jobs =
         Model.build_artifact_data datadir >>= fun build_env_right ->
         Model.build_artifact build_right.Builder_db.Build.uuid (Fpath.v "system-packages") conn >>=
         Model.build_artifact_data datadir >>= fun system_packages_right ->
-        resolve_artifact build_left.Builder_db.Build.main_binary conn >>= fun build_left_file ->
-        resolve_artifact build_right.Builder_db.Build.main_binary conn >>= fun build_right_file ->
+        resolve_artifact_size build_left.Builder_db.Build.main_binary conn >>= fun build_left_file_size ->
+        resolve_artifact_size build_right.Builder_db.Build.main_binary conn >>= fun build_right_file_size ->
         Model.job_name build_left.job_id conn >>= fun job_left ->
         Model.job_name build_right.job_id conn >|= fun job_right ->
-        (job_left, job_right, build_left, build_right, build_left_file,
-         build_right_file, switch_left, build_env_left, system_packages_left,
+        (job_left, job_right, build_left, build_right, build_left_file_size,
+         build_right_file_size, switch_left, build_env_left, system_packages_left,
          switch_right, build_env_right, system_packages_right))
     |> if_error "Internal server error"
-    >>= fun (job_left, job_right, build_left, build_right, build_left_file,
-              build_right_file, switch_left, build_env_left, system_packages_left,
+    >>= fun (job_left, job_right, build_left, build_right, build_left_file_size,
+              build_right_file_size, switch_left, build_env_left, system_packages_left,
              switch_right, build_env_right, system_packages_right) ->
     let env_diff = Utils.compare_env build_env_left build_env_right
     and pkg_diff = Utils.compare_pkgs system_packages_left system_packages_right
@@ -601,16 +598,17 @@ let routes ~datadir ~cachedir ~configdir ~expired_jobs =
     let switch_left = OpamFile.SwitchExport.read_from_string switch_left
     and switch_right = OpamFile.SwitchExport.read_from_string switch_right in
     let opam_diff = Opamdiff.compare switch_left switch_right in
-    (job_left, job_right, build_left, build_right, build_left_file, build_right_file, env_diff, pkg_diff, opam_diff)
+    (job_left, job_right, build_left, build_right, build_left_file_size, build_right_file_size, env_diff, pkg_diff, opam_diff)
     |> Lwt.return_ok
 
   in
 
   let compare_builds req =
     process_comparison req >>= fun
-    (job_left, job_right, build_left, build_right, build_left_file, build_right_file, env_diff, pkg_diff, opam_diff) ->
+    (job_left, job_right, build_left, build_right, build_left_file_size, build_right_file_size, env_diff, pkg_diff, opam_diff) ->
       match Dream.header req "Accept" with
         | Some accept when String.starts_with ~prefix:"application/json" accept ->
+          let file_size_json = Option.fold ~none:`Null ~some:(fun size -> `Int size) in
           let json_response =
             `Assoc [
               "left", `Assoc [
@@ -619,7 +617,7 @@ let routes ~datadir ~cachedir ~configdir ~expired_jobs =
                 "platform", `String build_left.platform;
                 "build_start_time", `String (Ptime.to_rfc3339 build_left.start);
                 "build_finish_time", `String (Ptime.to_rfc3339 build_left.finish);
-                "build_size", `Int (build_left_file.size)
+                "build_size", file_size_json build_left_file_size;
               ];
               "right", `Assoc [
                 "job", `String job_right;
@@ -627,7 +625,7 @@ let routes ~datadir ~cachedir ~configdir ~expired_jobs =
                 "platform", `String build_right.platform;
                 "build_start_time", `String (Ptime.to_rfc3339 build_right.start);
                 "build_finish_time", `String (Ptime.to_rfc3339 build_right.finish);
-                "build_size", `Int (build_right_file.size)
+                "build_size", file_size_json build_right_file_size;
               ];
               "env_diff", Utils.diff_map_to_json env_diff;
               "package_diff", Utils.diff_map_to_json pkg_diff;