ResourceTool work not propertly when change the bundle property

when i use the ResourceTool in a standalone situation, i found that when i configured the toolbox.xml as below(my resource files is study/velocity/resources/resource(_XXX).properties in my classpath)
<tools>
<toolbox scope="request">
<tool class="org.apache.velocity.tools.generic.ResourceTool"
bundles="study.velocity.resources.resource"
locale="en_US" />
</toolbox>
</tools>

it will not work properly, and throw an exception as below:

java.util.MissingResourceException: Can't find resource for bundle resources, key zh_CN

my template is as below:

Hi! My name is $text.test.user.name.
Oh! How are you $text.test.user.name? Where are you from?
I'm from $text.test.user.addr.
ok, $text.test.user.name. If I want to have a message with you, how do I contact you?
You can send me an email. My email address is $text.test.user.email.

but when i changed the toolbox.xml as below:
<tools>
<toolbox scope="request">
<tool key="res"
class="org.apache.velocity.tools.generic.ResourceTool"
bundles="study.velocity.resources.resource"
locale="en_US"/>
</toolbox>
</tools>

and changed the template as below:

Hi! My name is $res.test.user.name.
Oh! How are you $res.test.user.name? Where are you from?
I'm from $res.test.user.addr.
ok, $res.test.user.name. If I want to have a message with you, how do I contact you?
You can send me an email. My email address is $res.test.user.email.

It worked correctly.

so i don't know if it is a bug or some configuration error in my enviroment?