How to compose input object for "processAPI.executeUserTask" method

Hi to All,
I'm having difficulties in consuming method processAPI.executeUserTask(long userTaskInstanceId, Map<String,Serializable> inputs) via bonita java API.

I just can't format Serializable object/value for Map<String,Serializable> in a right way. I always get exception "cannot be assigned to COMPLEX type"

org.bonitasoft.engine.bpm.contract.ContractViolationException: USERNAME=walter.bates | Error while validating expected inputs: [[{attributeName: "aaaaaaaa", attributeValue: "bbbbbbbbb"}] cannot be assigned to COMPLEX type]