diff --git a/flake.nix b/flake.nix
index 0a74a2c..9b40204 100644
--- a/flake.nix
+++ b/flake.nix
@@ -97,7 +97,7 @@
 
                 cat > /var/lib/u/.env <<EOF
                 DATABASE_URL="sqlite:/var/lib/u/u.db"
-                U_PORT="${cfg.port}"
+                U_PORT="${toString cfg.port}"
                 U_FILES_DIR="/var/lib/u/files"
                 EOF