diff --git a/source/reference/operator/update/addToSet.txt b/source/reference/operator/update/addToSet.txt
index 19a2f7751d3..9fcb78ace3b 100644
--- a/source/reference/operator/update/addToSet.txt
+++ b/source/reference/operator/update/addToSet.txt
@@ -63,6 +63,10 @@ Behavior
   array as a *single* element. To add each element of the value
   separately, use :update:`$addToSet` with the :update:`$each`
   modifier. See :ref:`addToSet-modifiers` for details.
+  
+- If the value is an object, it will be deep-compared with every
+  element in the array. It is currently not possible to specify
+  particular keys to match against or ignore.
 
 .. _addToSet-modifiers: