diff -u b/core/modules/media_library/src/Form/MediaLibraryUploadForm.php b/core/modules/media_library/src/Form/MediaLibraryUploadForm.php
--- b/core/modules/media_library/src/Form/MediaLibraryUploadForm.php
+++ b/core/modules/media_library/src/Form/MediaLibraryUploadForm.php
@@ -128,7 +128,7 @@
         '#process' => array_merge(['::validateUploadElement'], $process, ['::processUploadElement']),
         '#allowed_types' => $this->getTypes(),
       ];
-      $this->typeResolver->alterFormElement($element, $form, $form_state);
+      $this->typeResolver->alterFormElement($form['upload'], $form, $form_state);
       $form['upload_help'] = [
         '#theme' => 'file_upload_help',
         '#description' => $this->t('Upload files here to add new media.'),
@@ -422,8 +422,10 @@
       'bundle' => $type,
       'name' => $file->getFilename(),
     ]);
-    $source_field = $media->bundle->entity->getSource()->getSourceFieldDefinition($type)->getName();
-    $media->set($source_field, $file->id());
+    $source_field = $media->bundle->entity->getSource()
+      ->getSourceFieldDefinition($media->bundle->entity)
+      ->getName();
+
     $location = $media
       ->set($source_field, $file->id())
       ->get($source_field)