diff --git a/modules/forgejo/options.nix b/modules/forgejo/options.nix
index 9b392af..b48a84d 100644
--- a/modules/forgejo/options.nix
+++ b/modules/forgejo/options.nix
@@ -42,4 +42,11 @@ args@{ config, lib, ... }:
       };
     };
   };
+
+  config.assertions = lib.mkIf config.aether.https [
+    {
+      assertion = !(builtins.isNull config.aether.acmeEmail);
+      message = "HTTPS support requires providing a contact email";
+    }
+  ];
 }
diff --git a/modules/options.nix b/modules/options.nix
index 944814d..5d606d0 100644
--- a/modules/options.nix
+++ b/modules/options.nix
@@ -12,8 +12,8 @@
   };
 
   acmeEmail = lib.mkOption {
-    type = lib.types.str;
-    default = "";
+    type = lib.types.nullOr lib.types.str;
+    default = null;
     description = "Email address for ACME.";
   };
 }