I need to generate a PDF file for 2-color commercial printing, with one spot color and the second color solid black.

I have the graphic elements in two separate layers. I have defined the spot color as 100% cyan, per the printer's suggestion. The black elements are defined as 100% K in the CMYK color model.

The problem is that when I export as a PDF, "there are combination colors in all separations", according to the printer. I tried both normal PDF (300 dpi) and PDF/X and got the same result. I also tried generating a PSD ... again, with the same result.

The printer needs a strict 2-color PDF. Does anyone have a recommendation of what settings might work? Hopefully, this isn't a new problem.

Thanks in advance for any help.