diff --git a/wp-includes/media.php b/wp-includes/media.php
index f30b7e14fc..ab33c88c04 100644
--- a/wp-includes/media.php
+++ b/wp-includes/media.php
@@ -367,7 +367,7 @@ function set_post_thumbnail_size( $width = 0, $height = 0, $crop = false ) {
  * @param string       $title Image description for the title attribute.
  * @param string       $align Part of the class name for aligning the image.
  * @param string|int[] $size  Optional. Image size. Accepts any registered image size name, or an array of
- *                           width and height values in pixels (in that order). Default 'medium'.
+ *                            width and height values in pixels (in that order). Default 'medium'.
  * @return string HTML IMG element for given image attachment
  */
 function get_image_tag( $id, $alt, $title, $align, $size = 'medium' ) {
@@ -377,7 +377,8 @@ function get_image_tag( $id, $alt, $title, $align, $size = 'medium' ) {
 
 	$title = $title ? 'title="' . esc_attr( $title ) . '" ' : '';
 
-	$class = 'align' . esc_attr( $align ) . ' size-' . esc_attr( $size ) . ' wp-image-' . $id;
+	$size_class = is_array( $size ) ? implode( 'x', $size ) : $size;
+	$class      = 'align' . esc_attr( $align ) . ' size-' . esc_attr( $size_class ) . ' wp-image-' . $id;
 
 	/**
 	 * Filters the value of the attachment's image tag class attribute.
@@ -2334,7 +2335,7 @@ function gallery_shortcode( $attr ) {
 		</style>\n\t\t";
 	}
 
-	$size_class  = sanitize_html_class( $atts['size'] );
+	$size_class  = sanitize_html_class( is_array( $atts['size'] ) ? implode( 'x', $atts['size'] ) : $atts['size'] );
 	$gallery_div = "<div id='$selector' class='gallery galleryid-{$id} gallery-columns-{$columns} gallery-size-{$size_class}'>";
 
 	/**
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 576dfb71eb..7ee177bc68 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.6-alpha-49127';
+$wp_version = '5.6-alpha-49128';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.