--- a/core/misc/collapse.js
+++ b/core/misc/collapse.js
@@ -81,7 +81,7 @@
       $('<span class="fieldset-legend-prefix element-invisible"></span>')
         .append($fieldset.hasClass('collapsed') ? Drupal.t('Show') : Drupal.t('Hide'))
         .prependTo($legend)
-        .after(' ');
+        .after('<span> </span>');

       // .wrapInner() does not retain bound events.
       var $link = $('<a class="fieldset-title" href="#"></a>')