s:actionError tag ignores theme=simple

Trying to use the <s:actionError> tag, we've encountered problems to display only the error message. It wasn't possible to simply display the error message string, without html injection.

Analyzing the struts-core package, we have found the following content inside the "actionerror.ftl":

<#if (actionErrors?exists && actionErrors?size > 0)>
<ul>
<#list actionErrors as error>
<li><span class="errorMessage">${error}</span></li>
</#list>
</ul>
</#if>

I don't know deeply the internals of the Struts 2 taglib API, but this seems to indicate that the <s:actionError> tag simply ignores the "theme" attribute, and always injects this template html.

Please help us telling how we can manage to display only the error message string, without this html template.

Thanks in advance.