paper-ripple is listening to click events outside of its container

I'm trying to create a simple ripple effect for a card in my app. It works great, but it is also responding to mouse events outside of its container:

Code looks like this:

<dom-module>
    ...
    <template>
        ...
        <paper-material>
            <div class="wrapper">...</div>
            <paper-ripple></paper-ripple>
        </paper-material>
        ...
    </template>
</dom-module>

I don't quite see why it is behaving this way. Any idea?

Added

This whole page is an independant dom-module. Apparently using paper-ripple in such a fashion causes this to happen. How do I fix this?

Copyright Notice：Content Author:「Derek 朕會功夫」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/30768536/paper-ripple-is-listening-to-click-events-outside-of-its-container