diff --git a/src/wp-includes/media.php b/src/wp-includes/media.php
index d4b43494c7..95bbca732e 100644
--- a/src/wp-includes/media.php
+++ b/src/wp-includes/media.php
@@ -896,7 +896,7 @@ function img_caption_shortcode( $attr, $content = null ) {
 		return $content;
 
 	if ( ! empty( $atts['id'] ) )
-		$atts['id'] = 'id="' . esc_attr( $atts['id'] ) . '" ';
+		$atts['id'] = 'id="' . esc_attr( sanitize_html_class( $atts['id'] ) ) . '" ';
 
 	$class = trim( 'wp-caption ' . $atts['align'] . ' ' . $atts['class'] );