Impossible to wrap the content in a span

I made plugin with a button than should wrap the selected text like that:

<h2><span>selection</span></h2>

The problem is than the span look like filter out. If I use em instead it works fine.

- no HTML filter in my text style.
- line break on
- use ckeditor

My JS:

invoke: function(data, settings, instanceId) {

        content = this._addWrapper(data.content);
        // Insert new content into the editor.
        if (typeof content != 'undefined') {
            Drupal.wysiwyg.instances[instanceId].insert(content);
        }

    },

    _addWrapper: function(htmlContent) {
        return '<h2><span>' + htmlContent + '</span></h2>';
    }