Indexing fails for search fields with role "image"

We've created a very simple article bundle based on the ContentBundle. So far it works great! Persisting new and existing entities in the database works fine. But indexing an entity fails due to an "array to string conversion".

After some debugging and hints from Alex in Slack, we've found that <tag name="sulu.search.field" role="image"/> seems to be the culprit. Removing this line will result in a successful indexing.

Related note: Using <tag name="sulu.search.field" role="image"/> with a content element on pages works fine.

Expected behavior

Entity is successfully persisted and indexed.

Actual behavior

Entity is successfully persisted in the database but indexing fails with the following exception:

ErrorException: Notice: Array to string conversion in /var/www/html/vendor/handcraftedinthealps/zendsearch/library/ZendSearch/Lucene/Storage/File/AbstractFile.php:341
Stack trace:
#0 /var/www/html/vendor/handcraftedinthealps/zendsearch/library/ZendSearch/Lucene/Index/SegmentWriter/AbstractSegmentWriter.php(229): ZendSearch\\Lucene\\Storage\\File\\AbstractFile->writeString(\'Array\')
#1 /var/www/html/vendor/handcraftedinthealps/zendsearch/library/ZendSearch/Lucene/Index/SegmentWriter/DocumentWriter.php(166): ZendSearch\\Lucene\\Index\\SegmentWriter\\AbstractSegmentWriter->addStoredFields(Array)
#2 /var/www/html/vendor/handcraftedinthealps/zendsearch/library/ZendSearch/Lucene/Index/Writer.php(231): ZendSearch\\Lucene\\Index\\SegmentWriter\\DocumentWriter->addDocument(Object(ZendSearch\\Lucene\\Document))
#3 /var/www/html/vendor/handcraftedinthealps/zendsearch/library/ZendSearch/Lucene/Index.php(1132): ZendSearch\\Lucene\\Index\\Writer->addDocument(Object(ZendSearch\\Lucene\\Document))
#4 /var/www/html/vendor/massive/search-bundle/Search/Adapter/ZendLuceneAdapter.php(172): ZendSearch\\Lucene\\Index->addDocument(Object(ZendSearch\\Lucene\\Document))
#5 /var/www/html/vendor/massive/search-bundle/Search/SearchManager.php(178): Massive\\Bundle\\SearchBundle\\Search\\Adapter\\ZendLuceneAdapter->index(Object(Sulu\\Bundle\\SearchBundle\\Search\\Document), \'massive_article...\')
#6 /var/www/html/vendor/sulu/content-bundle/Content/Application/ContentIndexer/ContentIndexer.php(60): Massive\\Bundle\\SearchBundle\\Search\\SearchManager->index(Object(Namespace\\Bundle\\ArticleBundle\\Entity\\ArticleDimensionContent))
#7 /var/www/html/vendor/sulu/content-bundle/Content/Application/ContentIndexer/ContentIndexer.php(49): Sulu\\Bundle\\ContentBundle\\Content\\Application\\ContentIndexer\\ContentIndexer->indexDimensionContent(Object(Namespace\\Bundle\\ArticleBundle\\Entity\\ArticleDimensionContent))
#8 /var/www/html/bundles/sulu-article-bundle/Controller/ArticleController.php(248): Sulu\\Bundle\\ContentBundle\\Content\\Application\\ContentIndexer\\ContentIndexer->index(Object(Namespace\\Bundle\\ArticleBundle\\Entity\\Article), Array)
#9 /var/www/html/vendor/symfony/http-kernel/HttpKernel.php(157): Namespace\\Bundle\\ArticleBundle\\Controller\\ArticleController->putAction(Object(Symfony\\Component\\HttpFoundation\\Request), 1)
#10 /var/www/html/vendor/symfony/http-kernel/HttpKernel.php(79): Symfony\\Component\\HttpKernel\\HttpKernel->handleRaw(Object(Symfony\\Component\\HttpFoundation\\Request), 1)
#11 /var/www/html/vendor/symfony/http-kernel/Kernel.php(195): Symfony\\Component\\HttpKernel\\HttpKernel->handle(Object(Symfony\\Component\\HttpFoundation\\Request), 1, true)
#12 /var/www/html/public/index.php(66): Symfony\\Component\\HttpKernel\\Kernel->handle(Object(Symfony\\Component\\HttpFoundation\\Request))
#13 {main}

Example

Here is our template example:

<?xml version="1.0" ?>
<template xmlns="http://schemas.sulu.io/template/template"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://schemas.sulu.io/template/template http://schemas.sulu.io/template/template-1.0.xsd">

    <key>default</key>

    <view>articles/default</view>
    <controller>Sulu\Bundle\WebsiteBundle\Controller\DefaultController::indexAction</controller>
    <cacheLifetime>604800</cacheLifetime>

    <meta>
        <title lang="en">Article</title>
        <title lang="de">Artikel</title>
    </meta>

    <properties>
        <property name="title" type="text_line" mandatory="true">
            <meta>
                <title lang="en">Title</title>
                <title lang="de">Titel</title>
            </meta>

            <params>
                <param name="headline" value="true"/>
            </params>

            <tag name="sulu.rlp.part"/>
            <tag name="sulu.search.field" role="title"/>
        </property>

        <property name="url" type="route">
            <meta>
                <title lang="en">Resourcelocator</title>
                <title lang="de">Adresse</title>
            </meta>

            <tag name="sulu.rlp"/>
            <tag name="sulu.search.field" role="url"/>
        </property>

        <property name="teaser_text" type="text_area">
            <meta>
                <title lang="en">Teaser Text</title>
                <title lang="de">Teaser Test</title>
            </meta>

            <tag name="sulu.search.field" role="description"/>
        </property>

        <property name="teaser_image" type="single_media_selection">
            <meta>
                <title lang="en">Teaser Image</title>
                <title lang="de">Teaser Bild</title>
            </meta>

            <tag name="sulu.search.field" role="image"/>
        </property>
    </properties>
</template>

sulu / SuluContentBundle