diff --git a/gpu/command_buffer/service/shared_image/compound_image_backing.cc b/gpu/command_buffer/service/shared_image/compound_image_backing.cc
index fce45e5cdb637..64ff7a2db5033 100644
--- a/gpu/command_buffer/service/shared_image/compound_image_backing.cc
+++ b/gpu/command_buffer/service/shared_image/compound_image_backing.cc
@@ -1601,6 +1601,12 @@ const std::vector<SkPixmap>& CompoundImageBacking::GetSharedMemoryPixmaps() {
   AutoLock auto_lock(this);
   auto* shm_backing = GetShmElement().GetBacking();
   DCHECK(shm_backing);
+  // SECURITY: When kUseCompoundImageBackingAsDefault is on, WrapExternalBacking
+  // wraps arbitrary backing types and gives them AccessStreamSet::All()
+  // (including kMemory). GetShmElement() then returns that wrapped backing
+  // here regardless of its actual type. Guard against the resulting type
+  // confusion in the static_cast below.
+  CHECK_EQ(shm_backing->GetType(), SharedImageBackingType::kSharedMemory);
 
   return static_cast<SharedMemoryImageBacking*>(shm_backing)->pixmaps();
 }