diff --git a/modules/nixos/services/calibre.nix b/modules/nixos/services/calibre.nix
index 698380b..814a110 100644
--- a/modules/nixos/services/calibre.nix
+++ b/modules/nixos/services/calibre.nix
@@ -30,8 +30,11 @@ in {
       match = [{ host = [ config.hostnames.books ]; }];
       handle = [{
         handler = "reverse_proxy";
-        upstreams =
-          [{ dial = "localhost:${config.services.calibre-web.listen.port}"; }];
+        upstreams = [{
+          dial = "localhost:${
+              builtins.toString config.services.calibre-web.listen.port
+            }";
+        }];
         headers.request.add."X-Script-Name" = [ "/calibre-web" ];
       }];
     }];
diff --git a/modules/nixos/services/grafana.nix b/modules/nixos/services/grafana.nix
index dd96cf5..3c22bb1 100644
--- a/modules/nixos/services/grafana.nix
+++ b/modules/nixos/services/grafana.nix
@@ -774,8 +774,10 @@ in {
                       {
                         matcher = {
                           id = "byName";
-                          options =
-                            "localhost:${config.services.calibre-web.listen.port}";
+                          options = "localhost:${
+                              builtins.toString
+                              config.services.calibre-web.listen.port
+                            }";
                         };
                         properties = [
                           {