diff --git a/lib/helpers/query/castUpdate.js b/lib/helpers/query/castUpdate.js
index 37a53b5a7e6..231da434e17 100644
--- a/lib/helpers/query/castUpdate.js
+++ b/lib/helpers/query/castUpdate.js
@@ -249,7 +249,8 @@ function walkUpdatePath(schema, obj, op, options, context, filter, pref) {
 
       let pathDetails = schema._getPathType(checkPath);
 
-      // If no schema type, check for embedded discriminators
+      // If no schema type, check for embedded discriminators because the
+      // filter or update may imply an embedded discriminator type. See #8378
       if (schematype == null) {
         const _res = getEmbeddedDiscriminatorPath(schema, obj, filter, checkPath);
         if (_res.schematype != null) {
diff --git a/lib/helpers/query/getEmbeddedDiscriminatorPath.js b/lib/helpers/query/getEmbeddedDiscriminatorPath.js
index 8214158377b..ff297ace6ff 100644
--- a/lib/helpers/query/getEmbeddedDiscriminatorPath.js
+++ b/lib/helpers/query/getEmbeddedDiscriminatorPath.js
@@ -45,6 +45,10 @@ module.exports = function getEmbeddedDiscriminatorPath(schema, update, filter, p
         discriminatorKey = filter[wrapperPath].$elemMatch[key];
       }
 
+      if (discriminatorValuePath in update) {
+        discriminatorKey = update[discriminatorValuePath];
+      }
+
       if (discriminatorKey == null || discriminators[discriminatorKey] == null) {
         continue;
       }