diff --git a/packages/compiler/src/backend/llvm/dyn.ts b/packages/compiler/src/backend/llvm/dyn.ts
index cf7578c02..ae4f12fb8 100644
--- a/packages/compiler/src/backend/llvm/dyn.ts
+++ b/packages/compiler/src/backend/llvm/dyn.ts
@@ -762,7 +762,9 @@ export class LlDyn {
       B.terminate(`ret i1 true`);
       B.startBlock(lNext);
     }
-    if (t.kind !== "dyn") {
+    // A union matches a capsule of another type through its arms below: a
+    // class capsule in a `C | undefined` field matches the `C` arm.
+    if (t.kind !== "dyn" && t.kind !== "union") {
       const view = this.viewFn(wrapped);
       this.host.declare(`declare void @scr_dyn_release_v(ptr)`);
       const kind = this.kindOf(B, "%d");
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6063-union-class-field-checks.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6063-union-class-field-checks.ts.json
new file mode 100644
index 000000000..9e48b2910
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6063-union-class-field-checks.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/6063-union-class-field-checks.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/6063-union-class-field-checks.ts b/tests/corpus/6063-union-class-field-checks.ts
new file mode 100644
index 000000000..426182b28
--- /dev/null
+++ b/tests/corpus/6063-union-class-field-checks.ts
@@ -0,0 +1,65 @@
+// A checked conversion of a dynamic value to a union of object types picks
+// the member whose fields match, including a field of type `C | undefined`
+// that holds an instance of the class C: workerData read as such a union,
+// and an unknown value asserted to it, give the instance or undefined.
+import { isMainThread, parentPort, Worker, workerData } from "node:worker_threads";
+import { publish } from "@scriptc/threads";
+
+class Job {
+  readonly name: string;
+  constructor(name: string) {
+    this.name = name;
+  }
+}
+
+interface WithJob {
+  readonly kind: "job";
+  readonly job: Job | undefined;
+}
+interface Plain {
+  readonly kind: "plain";
+  readonly note: string;
+}
+interface Counted {
+  readonly kind: "counted";
+  readonly count: number;
+  readonly last: Job | undefined;
+}
+type Data = WithJob | Plain | Counted;
+
+function describe(data: Data): string {
+  switch (data.kind) {
+    case "job":
+      return data.job === undefined ? "job none" : `job ${data.job.name}`;
+    case "plain":
+      return `plain ${data.note}`;
+    case "counted":
+      return `counted ${data.count} ${data.last?.name ?? "none"}`;
+  }
+}
+
+function run(data: Data): Promise<void> {
+  return new Promise((resolve) => {
+    const worker = new Worker(new URL(import.meta.url), { workerData: data });
+    worker.on("message", (message) => console.log("worker:", message));
+    worker.on("error", (error) => console.log("worker failed:", String(error)));
+    worker.on("exit", () => resolve());
+  });
+}
+
+if (isMainThread) {
+  const values: unknown[] = [
+    { kind: "job", job: new Job("local") },
+    { kind: "job", job: undefined },
+    { kind: "plain", note: "text" },
+    { kind: "counted", count: 2, last: new Job("tail") },
+  ];
+  for (const value of values) console.log("local:", describe(value as Data));
+  const shared = publish(new Job("shared"));
+  await run({ kind: "job", job: shared });
+  await run({ kind: "job", job: undefined });
+  await run({ kind: "plain", note: "worker" });
+  await run({ kind: "counted", count: 3, last: shared });
+} else {
+  parentPort!.postMessage(describe(workerData as Data));
+}
diff --git a/tests/harness/corpus-costs/6063-union-class-field-checks.ts.json b/tests/harness/corpus-costs/6063-union-class-field-checks.ts.json
new file mode 100644
index 000000000..bba381595
--- /dev/null
+++ b/tests/harness/corpus-costs/6063-union-class-field-checks.ts.json
@@ -0,0 +1 @@
+{ "plain": 4152, "san": 6474 }