diff --git a/config/prod.exs b/config/prod.exs
index 9794831..55b2db5 100644
--- a/config/prod.exs
+++ b/config/prod.exs
@@ -9,8 +9,15 @@ use Mix.Config
 # manifest is generated by the `mix phx.digest` task,
 # which you should run after static files are built and
 # before starting your production server.
+
+host =
+  System.get_env("HOSTNAME") ||
+    raise """
+    environment variable HOSTNAME is missing.
+    """
+
 config :planner, PlannerWeb.Endpoint,
-  url: [host: "planner.thermokar.st", port: 5000],
+  url: [host: host, port: 5000],
   cache_static_manifest: "priv/static/cache_manifest.json"
 
 # Do not print debug messages in production