diff --git a/frontend/packages/dashboard/src/schedules.js b/frontend/packages/dashboard/src/schedules.js
index 410567c0..0b196a0d 100644
--- a/frontend/packages/dashboard/src/schedules.js
+++ b/frontend/packages/dashboard/src/schedules.js
@@ -1072,8 +1072,11 @@ async function postScheduleCancel(id, targets) {
     const opts = {
       method: 'POST',
       headers: { 'Content-Type': 'application/json' },
+      // Always send a valid JSON body — cancel-all is `{}` (an empty body
+      // with a json content-type would 400 a strict extractor). Per-target
+      // cancel carries the list.
+      body: JSON.stringify(targets ? { targets } : {}),
     };
-    if (targets) opts.body = JSON.stringify({ targets });
     const resp = await fetch('/api/schedules/' + encodeURIComponent(id) + '/cancel', opts);
     if (!resp.ok) {
       const text = await resp.text().catch(() => '');