diff --git a/nix/modules/hive-matrix.nix b/nix/modules/hive-matrix.nix
index 85716052..dd125074 100644
--- a/nix/modules/hive-matrix.nix
+++ b/nix/modules/hive-matrix.nix
@@ -198,8 +198,10 @@ in
           # routing lands (#609 — `matrix.${hyperhiveDomain}`), drop
           # this override; upstream's `/` base-href is correct at the
           # root of a dedicated subdomain.
-          flutterBuildFlags =
-            (old.flutterBuildFlags or "") + " --base-href /matrix/";
+          flutterBuildFlags = (old.flutterBuildFlags or [ ]) ++ [
+            "--base-href"
+            "/matrix/"
+          ];
         });
         defaultText = lib.literalMD "`pkgs.fluffychat-web` rebuilt with `--base-href /matrix/` via `flutterBuildFlags`.";
         description = ''