diff --git a/js/src/builtin/TypedObject.h b/js/src/builtin/TypedObject.h
index ec902b22612..02af9c11769 100644
--- a/js/src/builtin/TypedObject.h
+++ b/js/src/builtin/TypedObject.h
@@ -1071,6 +1071,19 @@ IsTypedObjectClass(const Class *class_)
            class_ == &InlineOpaqueTypedObject::class_;
 }
 
+inline bool
+IsOpaqueTypedObjectClass(const Class *class_)
+{
+    return class_ == &OutlineOpaqueTypedObject::class_ ||
+           class_ == &InlineOpaqueTypedObject::class_;
+}
+
+inline const Class *
+GetOutlineTypedObjectClass(bool opaque)
+{
+    return opaque ? &OutlineOpaqueTypedObject::class_ : &OutlineTransparentTypedObject::class_;
+}
+
 inline bool
 IsSimpleTypeDescrClass(const Class* clasp)
 {
@@ -1103,7 +1116,7 @@ IsTypeDescrClass(const Class* clasp)
 inline bool
 TypedObject::opaque() const
 {
-    return is<OutlineOpaqueTypedObject>() || is<InlineOpaqueTypedObject>();
+    return IsOpaqueTypedObjectClass(getClass());
 }
 
 // Inline transparent typed objects do not initially have an array buffer, but
diff --git a/js/src/jit/IonBuilder.cpp b/js/src/jit/IonBuilder.cpp
index 74d379a2632..ac3dbc73230 100644
--- a/js/src/jit/IonBuilder.cpp
+++ b/js/src/jit/IonBuilder.cpp
@@ -7411,13 +7411,17 @@ IonBuilder::pushDerivedTypedObject(bool *emitted,
     current->add(derivedTypedObj);
     current->push(derivedTypedObj);
 
-    // Determine (if possible) the class/proto that `derivedTypedObj`
-    // will have. For derived typed objects, the class (transparent vs
-    // opaque) will be the same as the incoming object from which the
-    // derived typed object is, well, derived. The prototype will be
-    // determined based on the type descriptor (and is immutable).
+    // Determine (if possible) the class/proto that `derivedTypedObj` will
+    // have. For derived typed objects, the opacity will be the same as the
+    // incoming object from which the derived typed object is, well, derived.
+    // The prototype will be determined based on the type descriptor (and is
+    // immutable).
     types::TemporaryTypeSet *objTypes = obj->resultTypeSet();
-    const Class *expectedClass = objTypes ? objTypes->getKnownClass() : nullptr;
+    const Class *expectedClass = nullptr;
+    if (const Class *objClass = objTypes ? objTypes->getKnownClass() : nullptr) {
+        MOZ_ASSERT(IsTypedObjectClass(objClass));
+        expectedClass = GetOutlineTypedObjectClass(IsOpaqueTypedObjectClass(objClass));
+    }
     const TypedProto *expectedProto = derivedPrediction.getKnownPrototype();
     MOZ_ASSERT_IF(expectedClass, IsTypedObjectClass(expectedClass));