Error when trying to retrieve AzureDevOps WorkItemLinks using WIQL in C#

Question

I get this error:

{"Value cannot be null. (Parameter 'source')"}

when it tries to get result.

I have only just started using WIQL and C# - I have successfully run queries to retrieve workitems, first time trying to get workitemlinks.

string query = "Select * From WorkItemLinks" +
" WHERE Source.[System.TeamProject] = " + TEAM_PROJECT +
" AND Target.[System.TeamProject] = " + TEAM_PROJECT +
" AND [System.Links.LinkType] = 'System.LinkTypes.Hierarchy-Forward' " +
" AND Source.[System.Id] = '119943' " +
" AND Source.[System.State] = 'Active' ";

string[] fields = { "System.Id", "System.TeamProject", "System.State" };

var queryExecutor = new QueryExecutor(orgName, personalAccessToken);
Task<IList<WorkItem>> task = queryExecutor.QueryWorkItems(query, fields);
        
System.Runtime.CompilerServices.TaskAwaiter<IList<WorkItem>> awaiter = task.GetAwaiter();

IList<WorkItem> workItems = awaiter.GetResult();  // fails here