Index: src/wp-includes/widgets/class-wp-widget-media-image.php
===================================================================
--- src/wp-includes/widgets/class-wp-widget-media-image.php	(revision 42028)
+++ src/wp-includes/widgets/class-wp-widget-media-image.php	(working copy)
@@ -177,7 +177,7 @@
 			$attachment = get_post( $instance['attachment_id'] );
 		}
 		if ( $attachment ) {
-			$caption = $attachment->post_excerpt;
+			$caption = "";
 			if ( $instance['caption'] ) {
 				$caption = $instance['caption'];
 			}