Image editing API: Inpainting with masks not working as expected

I'm trying to use the image edit endpoint to replace specific parts of an image using a mask, but the results are ignoring my mask area.

response = client.images.edit(
    image=open("photo.png", "rb"),
    mask=open("mask.png", "rb"),
    prompt="A red sports car",
    n=1,
    size="1024x1024"
)

My mask is a PNG with the area to edit in transparent and the rest in black. But the entire image gets regenerated. Am I creating the mask wrong?