Index: src/wp-includes/media.php
===================================================================
--- src/wp-includes/media.php	(revision 44294)
+++ src/wp-includes/media.php	(working copy)
@@ -1569,7 +1569,7 @@
 		$id         = 'id="' . esc_attr( $atts['id'] ) . '" ';
 	}
 
-	if ( $atts['caption_id'] ) {
+	if ( isset($atts['caption_id']) ) {
 		$atts['caption_id'] = sanitize_html_class( $atts['caption_id'] );
 	} elseif ( $atts['id'] ) {
 		$atts['caption_id'] = 'caption-' . str_replace( '_', '-', $atts['id'] );