[2176] Using filter in style cased an error

using filter 'relativePath' is ok, but an error is cased in style

code:

<a class="activity-img" :href="activityItem.href_url|relativePath"
	:style="{'background-image':'url('+activityItem.image_src|relativePath+')'}" >
    <div class="activity-title-default">
	<div class="activity-title">{{ activityItem.title }}</div>
	<div class="activity-bg"></div>
    </div>
</a>

error:

vue.runtime.esm.js:430 [Vue warn]: Property or method "relativePath" is not defined on the instance  
 but referenced during render. Make sure to declare reactive data properties in the data option.