Issue launching in IE8

I was about to respond that I've changed nothing when I remembered that actually I have customised the toolbar. So I changed back to the Full toolbar and the problem went away. Here is my custom toolbar definition from the web page.

CKEDITOR.replace('editor1', 
   { toolbar:
      [
         ['Source', '-'],
         ['Format', 'Font', 'FontSize', 'TextColor'],
         ['Bold', 'Italic', '-', 'NumberedList', 'BulletedList', '-', 'Link'],
           ['Cut', 'Copy', 'Paste', 'PasteText', 'PasteFromWord'],
         ['Undo', 'Redo', '-', 'Find', 'Replace', '-', 'SelectAll', 'RemoveFormat'],
         ['Checkbox', 'Radio'],
          ['JustifyLeft', 'JustifyCenter', 'JustifyRight', 'JustifyBlock'],
          ['Image', 'HorizontalRule', 'Smiley', 'SpecialChar']
      ]
   });

That, however, is not the whole story. I also get the same error if I use the following:

CKEDITOR.replace('editor1', { toolbar : 'Basic' });

Any ideas?