diff --git a/lib/helpers/document/compile.js b/lib/helpers/document/compile.js
index 0868f24516c..1a2a0357d8b 100644
--- a/lib/helpers/document/compile.js
+++ b/lib/helpers/document/compile.js
@@ -2,6 +2,7 @@
 
 const documentSchemaSymbol = require('../../helpers/symbols').documentSchemaSymbol;
 const get = require('../../helpers/get');
+const internalToObjectOptions = require('../../options').internalToObjectOptions;
 const utils = require('../../utils');
 
 let Document;
@@ -142,6 +143,8 @@ function defineKey(prop, subprops, prototype, prefix, keys, options) {
           // Convert top-level to POJO, but leave subdocs hydrated so `$set`
           // can handle them. See gh-9293.
           v = v.$__parent.get(path);
+        } else if (v instanceof Document && !v.$__isNested) {
+          v = v.toObject(internalToObjectOptions);
         }
         const doc = this.$__[scopeSymbol] || this;
         doc.$set(path, v);