diff --git a/include/ui/GraphicBufferMapper.h b/include/ui/GraphicBufferMapper.h
index 8e93f72..001769f 100644
--- a/include/ui/GraphicBufferMapper.h
+++ b/include/ui/GraphicBufferMapper.h
@@ -39,7 +39,9 @@
 public:
     static inline GraphicBufferMapper& get() { return getInstance(); }
 
+    // This may NOT work on devices without a valid Gralloc2::Mapper.
     status_t registerBuffer(buffer_handle_t handle);
+
     status_t registerBuffer(const GraphicBuffer* buffer);
 
     status_t unregisterBuffer(buffer_handle_t handle);
diff --git a/libs/ui/GraphicBufferMapper.cpp b/libs/ui/GraphicBufferMapper.cpp
index b0ed2df..a3b6e18 100644
--- a/libs/ui/GraphicBufferMapper.cpp
+++ b/libs/ui/GraphicBufferMapper.cpp
@@ -63,7 +63,14 @@
     if (mMapper->valid()) {
         error = static_cast<gralloc1_error_t>(mMapper->retain(handle));
     } else {
+        // This always returns GRALLOC1_BAD_HANDLE when handle is from a
+        // remote process and mDevice is backed by Gralloc1On0Adapter.
         error = mDevice->retain(handle);
+        if (error == GRALLOC1_ERROR_BAD_HANDLE &&
+                mDevice->hasCapability(GRALLOC1_CAPABILITY_ON_ADAPTER)) {
+            ALOGE("registerBuffer by handle is not supported with "
+                  "Gralloc1On0Adapter");
+        }
     }
 
     ALOGW_IF(error != GRALLOC1_ERROR_NONE, "registerBuffer(%p) failed: %d",