Atlassian.Net SDK: Setting a Custom Field of type Issue fails with "Data was not an object"

[Amber Clark]

I am working on a piece of automation for the creation of Task issues.

C#, Visual Studio 2019, .Net Framework 4.7.2, Jira v8.9.0, Atlassian.SDK 12.1.1

One of the requirements from the team who manages our on-premise Jira solution is that every issue is linked to the parent issue.

Here's the custom field definition.

"customfield_12604":{
 "required":false,
 "schema":{
  "type":"issue",
  "custom":"com.codedpoetry.easylinks.easy-links:com.codedpoetry.easylinks.easy-links.configurable-single-issue-picker",
  "customId":12604
 },
 "name":"Child of - Link",
 "fieldId":"customfield_12604",
 "hasDefaultValue":false,
 "operations":["set","remove"]
}

Here's what my code look likes.

Issue issue = client.CreateIssue(projectKey);

issue.Summary = summary;
issue.Type = issueType;
issue.Assignee = assignedTo;
issue.Priority = priority;
issue["Initiative Link"] = initiativeLink;
issue.Components.Add(components);
issue.Labels.Add(label);
issue.FixVersions.Add(fixVersion);
issue["Target Date"] = dueDate.ToJsonFormat();
issue["Planview Project #"] = pvNumber;
issue.Description = summary + "\r\n\r\nCreated by S&E Jira Tasker";
issue["Child of - Link"] = "PM-803"; //  <-- This fails

issue.SaveChanges();

Here is the exception I get back from the server:

1. System.AggregateException: One or more errors occurred. --->
2. System.InvalidOperationException: Response Status Code: 400. Response Content: {"errorMessages":[],"errors":{"customfield_12604":"data was not an object"}}
3. at Atlassian.Jira.Remote.JiraRestClient.GetValidJsonFromResponse(IRestRequest request, IRestResponse response)
4. at Atlassian.Jira.Remote.JiraRestClient.<ExecuteRequestAsync>d__11.MoveNext()
5. --- End of stack trace from previous location where exception was thrown ---
6. at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
7. at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
8. at Atlassian.Jira.Remote.IssueService.<CreateIssueAsync>d__23.MoveNext()
9. --- End of inner exception stack trace ---
10. at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
11. at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
12. at System.Threading.Tasks.Task`1.get_Result()
13. at Atlassian.Jira.Issue.SaveChanges()
14. at Frontier.BssTool.SandEjiraTasker.JiraTask.<CreateIssue>d__0.MoveNext() in C:\code\TFS\Euro\SandEjiraTasker\SandEjiraTasker\JiraTask.cs:line 38
15. ---> (Inner Exception #0) System.InvalidOperationException: Response Status Code: 400. Response Content: {"errorMessages":[],"errors":{"customfield_12604":"data was not an object"}}
16. at Atlassian.Jira.Remote.JiraRestClient.GetValidJsonFromResponse(IRestRequest request, IRestResponse response)
17. at Atlassian.Jira.Remote.JiraRestClient.<ExecuteRequestAsync>d__11.MoveNext()
    --- End of stack trace from previous location where exception was thrown ---
18. at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
19. at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
20. at Atlassian.Jira.Remote.IssueService.<CreateIssueAsync>d__23.MoveNext()<---

Clearly I'm doing something wrong but I can't find documentation on how to do it right. Any pointers would be greatly appreciated.