diff --git a/src/wp-includes/media.php b/src/wp-includes/media.php
index e037a0608b..d365e0b1e1 100644
--- a/src/wp-includes/media.php
+++ b/src/wp-includes/media.php
@@ -2361,6 +2361,10 @@ add_shortcode( 'caption', 'img_caption_shortcode' );
  * @return string HTML content to display the caption.
  */
 function img_caption_shortcode( $attr, $content = '' ) {
+	if ( ! $attr ) {
+		$attr = array();
+	}
+
 	// New-style shortcode with the caption inside the shortcode with the link and image tags.
 	if ( ! isset( $attr['caption'] ) ) {
 		if ( preg_match( '#((?:<a [^>]+>\s*)?<img [^>]+>(?:\s*</a>)?)(.*)#is', $content, $matches ) ) {