diff --git a/xpcom/base/nsAgg.h b/xpcom/base/nsAgg.h
index a29879ef3539..aab49d805bfd 100644
--- a/xpcom/base/nsAgg.h
+++ b/xpcom/base/nsAgg.h
@@ -63,7 +63,12 @@ private:                                                                    \
                                                                             \
     friend class Internal;                                                  \
                                                                             \
-    nsISupports*        fOuter;                                             \
+    nsISupports* MOZ_UNSAFE_REF("fOuter can either point to fAggregated "   \
+                                "or to an outer object, and the safety "    \
+                                "of this reference depends on the exact "   \
+                                "lifetime semantics of the AddRef/Release " \
+                                "functions created by these macros.")       \
+                        fOuter;                                             \
     Internal            fAggregated;                                        \
                                                                             \
 public:                                                                     \