diff --git a/nix/host-modules/swarm-otel.nix b/nix/host-modules/swarm-otel.nix
index 2493f91c..d1e16055 100644
--- a/nix/host-modules/swarm-otel.nix
+++ b/nix/host-modules/swarm-otel.nix
@@ -700,6 +700,39 @@ in
           leaving both log destinations unset.
         '';
       }
+      {
+        # The assertion above covers *which* units to collect; this one
+        # covers whether there is a journal on disk to collect them from.
+        # A `bindMounts` entry never creates its `hostPath`, and unlike
+        # the CA bind source above there is no unit to order after — the
+        # directory exists because journald was told to store
+        # persistently, or not at all. `auto` is deliberately not
+        # rejected: it uses the directory when it exists, and eval cannot
+        # see whether it does.
+        assertion =
+          !collectLogs
+          || !(lib.elem config.services.journald.storage [
+            "volatile"
+            "none"
+          ]);
+        message = ''
+          The swarm collector is configured to ship journal logs, but
+          services.journald.storage is
+          "${config.services.journald.storage}" on this host.
+
+          journald only writes ${hostJournalDir} when it stores
+          persistently: with "volatile" the journal lives in
+          /run/log/journal, and with "none" there is none at all. This
+          collector's journald receiver reads ${hostJournalDir} and the
+          container bind-mounts that path, so the collector would not
+          start at all — nixos-container refuses to start when a bind
+          source is missing.
+
+          Set services.journald.storage = "persistent" (the NixOS
+          default), or turn off log collection by leaving both log
+          destinations unset.
+        '';
+      }
       {
         # Without a roster there are no receivers at all, so this
         # collector would listen on nothing while looking configured.