Spring WS: HttpMediaTypeNotAcceptableException: Could not find acceptable representation

I have the following Spring 4 WS controller:

@RestController
@RequestMapping("/ws/import")
@Scope("request")
public class ImportController {
    private static final Logger log = LoggerFactory.getLogger(ImportController.class);
    @Autowired private ContactsService contactsService;
    @Autowired private User user;

    @RequestMapping("/files")
    @ResponseBody
    public List<String> imports() {
        return contactsService.getImportFiles();
    }

    @RequestMapping("/sample/{file}")
    @ResponseBody
    public Sample sample(@PathVariable("file") String file) {
        log.debug("Sampling {}", file);
        return new Sample(100);
    }
}

All of my WS requests are working just fine, other than #sample. This is the only one that has both a path variable and a response body. For some reason it's throwing:

org.springframework.web.HttpMediaTypeNotAcceptableException: 
Could not find acceptable representation

It's binding fine as I see the log message as expected. It's on the response that it's having issues.

Here's my .xml config:

<mvc:annotation-driven>
    <mvc:message-converters register-defaults="true">
        <bean class="org.springframework.http.converter.StringHttpMessageConverter">
            <property name="supportedMediaTypes">
                <util:list value-type="java.lang.String">
                    <value>text/plain;charset=UTF-8</value>
                    <value>application/json;charset=UTF-8</value>
                </util:list>
            </property>
        </bean>
        <bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter" />
    </mvc:message-converters>
</mvc:annotation-driven>

It's driving me nuts!