diff --git a/tests/test_interactive_pane_js.py b/tests/test_interactive_pane_js.py
index 1d3dc6c0..be633033 100644
--- a/tests/test_interactive_pane_js.py
+++ b/tests/test_interactive_pane_js.py
@@ -830,6 +830,16 @@ if (c.includes("promote"))
 c = run(makeEl({ hasAttribute: (a) => a === "aria-busy" }), false, queued);
 if (c.includes("promote"))
   throw new Error("dismiss-in-flight chip must be left to its DELETE verdict: " + c);
+// Null / non-object 2xx body (a misbehaving proxy answering `200 null`): the
+// helper normalizes it to {} so neither call site guards — it must fall through
+// to the unknown/"ok" arm and SETTLE the optimistic chip (promote), never throw
+// and strand a delivered message as a connection error.  (The no-op
+// consumeAttachments stub cannot prevent this: data.attached_ids is evaluated to
+// build the :639 call args, so against unfixed code this line throws and crashes
+// the harness.)
+c = run(makeEl(), false, null);
+if (!c.includes("promote"))
+  throw new Error("null body must settle via unknown-ok, not throw: " + c);
 console.log("settle matrix OK");
 """,
         encoding="utf-8",
diff --git a/turnstone/console/static/coordinator/coordinator.js b/turnstone/console/static/coordinator/coordinator.js
index e6da753e..6a592c97 100644
--- a/turnstone/console/static/coordinator/coordinator.js
+++ b/turnstone/console/static/coordinator/coordinator.js
@@ -2068,7 +2068,7 @@ function createCoordinatorPane(root, wsId, opts) {
         // queue_full, attachments_busy, cross_user, unknown-ok) lives in
         // the shared helper — ONE settle matrix for both panes; see
         // settleSendResponse's contract for the arm semantics.
-        settleSendResponse(queue, data || {}, {
+        settleSendResponse(queue, data, {
           queuedEl,
           optimisticEl,
           isBusy,
diff --git a/turnstone/shared_static/composer_queue.js b/turnstone/shared_static/composer_queue.js
index 0d421ff6..7922cbdf 100644
--- a/turnstone/shared_static/composer_queue.js
+++ b/turnstone/shared_static/composer_queue.js
@@ -544,7 +544,15 @@ export function parsePriority(text) {
 //   busy / attachments_busy / cross_user_interjection / unknown-ok —
 //     the panes' historical shapes, verbatim.
 export function settleSendResponse(queue, data, ctx) {
-  var status = data && data.status;
+  // Normalize a null / non-object 2xx body once, here at the shared
+  // chokepoint, so neither pane's call site has to guard it (interactive
+  // passed bare `data`, the coordinator passed `data || {}` — the divergence
+  // this helper exists to erase).  In-tree /send always returns an object, so
+  // this only hardens against a misbehaving proxy answering e.g. `200 null`;
+  // without it the unknown/"ok" fall-through below would deref
+  // data.attached_ids and surface a delivered message as a connection error.
+  data = data || {};
+  var status = data.status;
   if (status === "queued" && data.msg_id) {
     var queuedEl = ctx.queuedEl;
     if (!queuedEl && data.deferred) {