diff --git a/lib/helpers/query/castUpdate.js b/lib/helpers/query/castUpdate.js
index bf60e5d11b7..1ee20fb41f2 100644
--- a/lib/helpers/query/castUpdate.js
+++ b/lib/helpers/query/castUpdate.js
@@ -203,6 +203,13 @@ function walkUpdatePath(schema, obj, op, options, context, filter, pref) {
       // watch for embedded doc schemas
       schematype = schema._getSchema(prefix + key);
 
+      if (schematype == null) {
+        const _res = getEmbeddedDiscriminatorPath(schema, obj, filter, prefix + key);
+        if (_res.schematype != null) {
+          schematype = _res.schematype;
+        }
+      }
+
       if (op !== '$setOnInsert' &&
           handleImmutable(schematype, strict, obj, key, prefix + key, context)) {
         continue;