How to use parameters in error messages using <s:text> in Struts 2?

When I try to put some dynamic parts in my texts to be used by <s:text> tag in Struts 2, these parts are replaced by the params that I defined.

Here is how I write my sentences in my file.properties:

my_error=The event {0} doesn't exist

Here is how I try to display it:

<s:text name="my_error">
    <s:param>Event01</s:param>
</s:text>

But in the result, the expression {0} is not replaced and I have no error in the log. What's wrong?

I really can't understand. I picked this example, so I have this in my jsp file:

<s:text name="msg.error">
    <s:param >Event01</s:param>
</s:text>
<br />
<s:text name="name.msg.param" >
    <s:param >mkyong</s:param>
</s:text>

and this in my .properties:

msg.error = This event doesn't exist: {0}
name.msg.param = This is a message from properties file - param : {0}

But the result is:

This event does not exist: {0}
This is a message from properties file - param : mkyong

I do not manage to find a real difference.