[TYPO3-mvc] problem with fed.contains viewhelper :: "Cannot cast object of type "Tx_Extbase_Persistence_ObjectStorage" to string."

[Roland]

edited to add: this is the not working snippet:

<fed:contains haystack="{partner.partnercategories}" needle="7">
     <fed:then>
         <dd>{partner.email}</dd>
     </fed:then>
</fed:contains>

{partner.partnercategories} instead of {partner.partnercategory}