xml parser error when rendering api call from twig template

[Ghkuijer]

When I'm rendering an API call from twig template I get the error:
An Exception was thrown while handling: Xml parsing error "Input is not proper UTF-8, indicate encoding ! Bytes: 0xE2 0xE3 0xCF 0xD3 "

If using the API Doc there is now problem in making the API call.
Code for calling is:

{{ render(controller('ShareworksProjectBundle:Items:getItems', {'slug': project.slug, '_format': 'pdf'})) }}


Is there something I'm doing wrong?

thx in advance