why is my createValidationResult wrong? And why is the ErrorCollection empty?

I'm trying to create subTasks to an Issue.

In my code I create the Issue first and then I try to create SubTasks via issueService.validateSubTaskCreate(.....) and so on.

when I run my plugin it creates the normal Issue just like I intended. but gives me an exeption:

java.lang.IllegalStateException: You can not create an issue with an invalid validation result.

I've tried different things up until now. I tried getErrorCollection() on the validationResult, which came back empty. But when I tried isValid() it returned false and I have no idea why since I did everything like I did with the main Issue

here's what I did:

public List<IssueInputParameters> subTaskParameters(Issue newIssue, Issue oldIssue){
    	List<IssueInputParameters> newParams = new ArrayList();
    	List<Issue> currentSubTasks = (List<Issue>) oldIssue.getSubTaskObjects();
    	
    	for(Issue issue : currentSubTasks){
    		IssueInputParameters input = issueService.newIssueInputParameters();
    		
    		input.setDescription(issue.getDescription());
    		input.setIssueTypeId(issue.getIssueTypeObject().getId());
    		input.setProjectId(issue.getProjectObject().getId());
   			input.setSummary(issue.getSummary());
   			input.setAssigneeId(getLoggedInUser().getDisplayName());
   			newParams.add(input);
    	}
    	
    	return newParams;
    }
    public void createSubTasks(Issue newIssue, Issue oldIssue){
    	List<IssueInputParameters> subTaskParams = subTaskParameters(newIssue, oldIssue);
    	
    	for(IssueInputParameters params : subTaskParams){
        	CreateValidationResult createValidationResult = issueService.validateSubTaskCreate(getLoggedInUser(), newIssue.getId(), params);
        	IssueResult result = issueService.create(getLoggedInUser(), createValidationResult);
           	}
    }

 Thanks beforehand.