Filtering tasks by task name (Form Key) using WorkflowTaskQuery

[Tabu]

Hi,
I wanted to filter the tasks using setTaskName which is the method of WorkflowTaskQuery class. I've tried it by using the below code as it requires QName as its parameter, but it didn't provide me with any of the result. I got an empty response.

String c1 = "aclrqreparePo";
QName fk;
fk = QName.createQName(c1, namespaceService);
WorkflowTaskQuery taskQuery = new WorkflowTaskQuery();
taskQuery.setActive(null);
taskQuery.setWorkflowDefinitionName(workflowName);
taskQuery.setTaskState(state);
taskQuery.setTaskName(fk);
allTasks = workflowService.queryTasks(taskQuery);

Please do provide me a solution. Any help would be appreciated
Thank you!