How to Filter empty date field in SF analytics api?

Using the analytics rest api we need to add a filter to a date field to get only the records where this field is empty. In json: { "column": "Account.Date_Field__c", "operator": "equals", "value": "" },

This filter is also shown if you fetch the report meta data from SF, so the syntax should be correct.

Till the rollout of Summer 14 release it was working as expected, after the release change we get the error message "Filter the date in the correct format. Accepted formats are yyyy-MM-dd'T'HH:mm:ss'Z' and yyyy-MM-dd.]" I checked to use null, but this leads to an error "574902135-2760 (-2136086741)"

Any ideas how to filter on empty date field?