Question: plot tickmarks error


I'm getting an error when I try to

plot(sin(x), x = -1..1, color = darkblue,tickmarks=[spacing(0.25),subticks=2]);

The error is
Error, (in plot) expecting option tickmarks to be of type list({"default", "piticks", "decimalticks", "pidefault", nonnegint, list(realcons), list(realcons = anything), spacing(realcons), spacing(realcons, realcons)}) but received [spacing(.25), subticks = 2]

What exactly am I doing wrong? After all, [...] is a list.

Even more confusingly, tickmarks=[[spacing(0.25),subticks=2],default] doesn't work either.

Any help appreciated!

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