I have created an image in photoshop 7 and am now attempting to save it as a png. I click "Save As" and am presented with a save file dialog with *.PSD selected by default. If I then select *.PNG a yellow triangle with a black exclamation mark appears next to a greyed out box called "Layers" and the message "file must be saved as a copy with this selection" appears. This is confusing me because the image is comprised of only a single layer.

How do I resolve this?