diff --git a/packages/compiler/src/frontend/lowering/call-signatures.ts b/packages/compiler/src/frontend/lowering/call-signatures.ts
index 9d9c9668c..557b1a462 100644
--- a/packages/compiler/src/frontend/lowering/call-signatures.ts
+++ b/packages/compiler/src/frontend/lowering/call-signatures.ts
@@ -408,7 +408,15 @@ export function paramShapes(
       return { type: mapped, mode: "rest" };
     }
     if (declParam.initializer) return defaultParameterShape(lowerer, declParam, mapped);
-    if (declParam.questionToken && !lowerer.bareUndefinedArmedUnion(mapped)) {
+    // A checked-value slot holds undefined directly, so an omitted
+    // argument needs no union arm (paramShape's declared-signature rule;
+    // an erased type parameter and a class-implemented interface view
+    // both map here).
+    if (
+      declParam.questionToken &&
+      !lowerer.bareUndefinedArmedUnion(mapped) &&
+      mapped.kind !== "dyn"
+    ) {
       lowerer.unsupported(
         "SC1090",
         declParam,
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6340-prototype-reflection-optional-view-param.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6340-prototype-reflection-optional-view-param.ts.json
new file mode 100644
index 000000000..89aef0f85
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6340-prototype-reflection-optional-view-param.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/6340-prototype-reflection-optional-view-param.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/6340-prototype-reflection-optional-view-param.ts b/tests/corpus/6340-prototype-reflection-optional-view-param.ts
new file mode 100644
index 000000000..58e21164d
--- /dev/null
+++ b/tests/corpus/6340-prototype-reflection-optional-view-param.ts
@@ -0,0 +1,42 @@
+// A generic method whose optional parameter is an interface view (or an
+// erased type parameter) reflects through the prototype of an instance
+// that crossed into checked-dynamic code, and calls directly with the
+// argument omitted.
+interface Handle {
+  close(): string;
+}
+class Plain implements Handle {
+  close(): string {
+    return "closed";
+  }
+}
+class Registry {
+  private current: Handle | null = null;
+  use(handle: Handle | null): void {
+    this.current = handle;
+  }
+  wrap<F>(fn: F, handle?: Handle | null): F {
+    if (handle) this.current = handle;
+    return fn;
+  }
+  tag<T>(value: T, label?: T): string {
+    return `${String(value)}:${label === undefined ? "none" : String(label)}`;
+  }
+  size(): number {
+    return this.current === null ? 0 : 1;
+  }
+}
+
+const registry = new Registry();
+const inc = registry.wrap((x: number) => x + 1);
+console.log(inc(1), registry.size());
+const dec = registry.wrap((x: number) => x - 1, new Plain());
+console.log(dec(1), registry.size());
+console.log(registry.tag(1), registry.tag("a", "b"));
+
+const boxed: unknown = registry;
+const proto = Object.getPrototypeOf(boxed);
+console.log(Object.getOwnPropertyNames(proto).join(","));
+console.log(typeof proto.wrap, typeof proto.tag, proto.size.call(registry));
+const loose: any = registry;
+console.log(loose.size(), proto.tag.call(registry, 2), proto.tag.call(registry, 2, 3));