Texture2D Submission Throwing 404?

[taylorbrown2686]

When I submit an image-based request:

public async Task<string> GetImageBasedReply(string prompt, Texture2D image)
        {
            ChatSession.chatMessages.Add(new Message(Role.System, new List<Content> {
                new Content(ContentType.Text, "Use this image for context to the prompt."),
                image
            }));
            ChatSession.chatMessages.Add(new Message(Role.User, prompt));

            ChatRequest request = new ChatRequest(ChatSession.chatMessages, model: "gpt-4-vision-preview", maxTokens: 200);
            ChatResponse response = await ChatSession.client.ChatEndpoint.GetCompletionAsync(request);

            if (response.Choices is not null && response.Choices.Count > 0)
            {
                var gptMessage = response.Choices[0].Message;

                print("GPT Response: " + gptMessage.Content.ToString());

                ChatSession.chatMessages.Add(gptMessage);

                return gptMessage.Content.ToString();
            }
            else
            {
                return null;
            }
        }

It throws a [POST:404] RestException, simply saying it 'Failed' with no additional information. The error itself shows the preprompt given along with a long string (Base64 assumed?) that is truncated for length.

What exactly is the '404' referring to, and how can I resolve it?