diff --git a/nixos/modules/services/web-apps/mastodon.nix b/nixos/modules/services/web-apps/mastodon.nix
index 3e1286dc475d..cf5329be489c 100644
--- a/nixos/modules/services/web-apps/mastodon.nix
+++ b/nixos/modules/services/web-apps/mastodon.nix
@@ -587,6 +587,13 @@ in {
             <option>services.mastodon.smtp.authenticate</option> is enabled.
         '';
       }
+      {
+        assertion = 1 == builtins.length
+          (lib.mapAttrsToList
+            (_: v: builtins.elem "scheduler" v.jobClasses || v.jobClasses == [ ])
+            cfg.sidekiqProcesses);
+        message = "There must be one and only one Sidekiq queue in services.mastodon.sidekiqProcesses with jobClass \"scheduler\".";
+      }
     ];
 
     environment.systemPackages = [ mastodonTootctl ];