Select text and apply style

I wany to select several different words and apply single style to them. I am using code found at stackoverflow:

// Note: your instance name may differ.
var editor = CKEDITOR.instances.editor1, 
    selection = editor.getSelection(),
    root = selection.root,
    textNodes = [],
    ranges = [],
    range, text, index;

function getTextNodes( element ) {
    var children = element.getChildren(),
        child;

    for ( var i = children.count(); i--; ) {
        child = children.getItem( i );
        if ( child.type == CKEDITOR.NODE_ELEMENT ) 
            getTextNodes( child );
        else if ( child.type == CKEDITOR.NODE_TEXT )
            textNodes.push( child );
    }
}

// Recursively search for text nodes starting from root.
// You may want to search a specific branch starting from other element.
getTextNodes( root ); 

// Iterate over found text nodes. If some contains
// phrase "the", create a range that selects this word.
for ( i = textNodes.length; i--; ) {
   text = textNodes[ i ];
   index = text.getText().indexOf( 'the' );
   if ( index > -1 ) {
      range = editor.createRange();
      range.setStart( text, index );
      // Note: 3 is fixed length of "the". You may want to change it.
      range.setEnd( text, index + 3 ); 
      ranges.push( range );
   }  
}

// Select all ranges "containing" phrase "the".
selection.selectRanges( ranges );

editor.execCommand('bold');

I want to have all the selected words in bold and I execute editor.execCommand('bold');, but I get an exception: Uncaught IndexSizeError: Index or size was negative, or greater than the allowed value.

How can I apply styles to selected items?