Gemini Pro Vision API error (Unable to submit request because it has more than one contents field)

[pq111]

Hello, I am using the gemini-pro-vision LLM with image inputs, but I get the following 400 error:

Unable to submit request because it has more than one contents field but model gemini-pro-vision only supports one. Remove all but one contents and try again.. Learn more: https://cloud.google.com/vertex-ai/generative-ai/docs/model-reference/gemini

This is a fake example of the contents part of my object:

contents: [{
   "role": "USER",
    "parts": [
       {"text": "what do you see in this image"},
       { "inlineData" : "(insert base64 image here)" }
    ]
}, {
    "role": "MODEL",
    "parts": [
        {"text": "be as descriptive as possible"}
    ]
}]

I cannot find any information in the API documentation about this. For the record, if I switch "MODEL" to be "USER" it works without error despite there being more than one element in the contents array. Also, if I don't use image inputs at all but still use MODEL, it also works without an error. It seems to only happen if I have a contents array with `inlineData`, more than one element. and more than one role. Is there a way around this or documentation somewhere explaining?