Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 22839)
+++ wp-includes/functions.php	(working copy)
@@ -2273,7 +2273,11 @@
  * @param mixed $response Variable (usually an array or object) to encode as JSON, then print and die.
  */
 function wp_send_json( $response ) {
-	@header( 'Content-Type: application/json; charset=' . get_option( 'blog_charset' ) );
+	if ( strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE') !== false )
+		@header( 'Content-Type: text/html;' );
+	else
+		@header( 'Content-Type: application/json; charset=' . get_option( 'blog_charset' ) );
+
 	echo json_encode( $response );
 	if ( defined( 'DOING_AJAX' ) && DOING_AJAX )
 		wp_die();