What causes "One or more of the requests is null." in jstedfast/MailKit?

Inspect the requests list before calling and remove/replace any null elements

jstedfast/MailKit · error · ArgumentException

One or more of the requests is null.

Error message

One or more of the requests is null.

What it means

ImapFolder.SetAnnotations/annotation APIs validate every entry in the requests list before queueing commands. If any element of the IList<IAnnotationRequest> is null, an ArgumentException is thrown. This is fail-fast argument validation so no partial IMAP work is issued.

Solutions

Inspect the requests list before calling and remove/replace any null elements
Ensure the code that creates each request object actually instantiates it (no null returns)
If entries are optional, filter them: requests.RemoveAll(r => r == null) or requests.Where(r => r != null)
Wrap the call in try/catch (ArgumentException) and report which list index was null

Example fix

// before
var requests = new List<IAnnotationRequest> { getRequest(id) }; // getRequest may return null
folder.SetAnnotations(requests);
// after
var requests = new List<IAnnotationRequest> { getRequest(id) ?? throw new InvalidOperationException("no annotation request for " + id) };
folder.SetAnnotations(requests);

Defensive patterns

Strategy: validation

Validate before calling

if (requests == null || requests.Any(r => r == null))
    throw new InvalidOperationException("all annotation requests must be non-null");

Type guard

bool AllNonNull<T>(IEnumerable<T> items) => items != null && !items.Any(i => i is null);

Try / catch

try { folder.SetAnnotations(requests); }
catch (ArgumentException ex) { log.LogError(ex, "Null annotation request in list"); }

Prevention

Validate list contents at construction time
Avoid factories that return null; use exceptions or Result types
Use nullable-reference-analysis warnings to catch null flow

When it happens

Trigger: Calling an annotations API (e.g. SetAnnotations/Store with annotation requests) passing a list where one or more elements are null, e.g. new IAnnotationRequest[] { request1, null }.

Common situations: Building request arrays dynamically and appending null from failed lookups; deserializing request lists from config/JSON where entries are missing; LINQ Select that returns null for some items.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/64aa38fd6fba9f16. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/Net/Imap/ImapFolder.cs:4607

		{
			var ic = QueueAppendCommand (options, request, cancellationToken);

			await Engine.RunAsync (ic).ConfigureAwait (false);

			return ProcessAppendResponse (ic);
		}

		void ValidateArguments (FormatOptions options, IList<IAppendRequest> requests)
		{
			if (options == null)
				throw new ArgumentNullException (nameof (options));

			if (requests == null)
				throw new ArgumentNullException (nameof (requests));

			for (int i = 0; i < requests.Count; i++) {
				if (requests[i] == null)
					throw new ArgumentException ("One or more of the requests is null.");

				var annotations = requests[i].Annotations;
				if (annotations != null && annotations.Count > 0 && (Engine.Capabilities & ImapCapabilities.Annotate) == 0)
					throw new NotSupportedException ("One ore more requests included annotations but the IMAP server does not support annotations.");
			}

			CheckState (false, false);
		}

		ImapCommand QueueMultiAppendCommand (FormatOptions options, IList<IAppendRequest> requests, CancellationToken cancellationToken)
		{
			var format = CreateAppendOptions (options);
			var builder = new StringBuilder ("APPEND %F");
			var list = new List<object> {
				this
			};

			for (int i = 0; i < requests.Count; i++) {

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