diff --git a/flake.nix b/flake.nix
index a6a21a1..4eea35d 100644
--- a/flake.nix
+++ b/flake.nix
@@ -113,7 +113,7 @@
                   ];
                   Environment = [
                     "ROCKET_ADDRESS=${cfg.address}"
-                    "ROCKET_PORT=${cfg.port}"
+                    "ROCKET_PORT=${builtins.toString cfg.port}"
                     "ROCKET_PUBLIC_URL=${cfg.public_url}"
                   ];
                 };