Index: src/wp-admin/includes/misc.php
===================================================================
--- src/wp-admin/includes/misc.php	(revision 40871)
+++ src/wp-admin/includes/misc.php	(working copy)
@@ -788,9 +788,16 @@
 				'text' => sprintf( __( '%s has taken over and is currently editing.' ), $user->display_name )
 			);
 
-			if ( $avatar = get_avatar( $user->ID, 64 ) ) {
-				if ( preg_match( "|src='([^']+)'|", $avatar, $matches ) )
-					$error['avatar_src'] = $matches[1];
+			$avatar_url_args = array(
+				'size' => 64
+			);
+			if ( $avatar_url = get_avatar_url( $user->ID, $avatar_url_args ) ) {
+				$error['avatar_src'] = $avatar_url;
+				$error['avatar_srcset'] = sprintf('%s 2x', esc_attr( get_avatar_url( $user->ID,
+							array_merge( $avatar_url_args,  array( 'size' => $avatar_url_args['size'] * 2 ) )
+						)
+					)
+				);
 			}
 
 			$send['lock_error'] = $error;
Index: src/wp-admin/js/post.js
===================================================================
--- src/wp-admin/js/post.js	(revision 40871)
+++ src/wp-admin/js/post.js	(working copy)
@@ -200,7 +200,9 @@
 					}
 
 					if ( received.lock_error.avatar_src ) {
-						avatar = $( '<img class="avatar avatar-64 photo" width="64" height="64" alt="" />' ).attr( 'src', received.lock_error.avatar_src.replace( /&amp;/g, '&' ) );
+						avatar = $( '<img class="avatar avatar-64 photo" width="64" height="64" alt="" />' )
+                            .attr( 'srcset', received.lock_error.avatar_srcset.replace( /&amp;/g, '&' ) )
+							.attr( 'src', received.lock_error.avatar_src.replace( /&amp;/g, '&' ) );
 						wrap.find('div.post-locked-avatar').empty().append( avatar );
 					}