Can the RS pin for a 16x2 character LCD be controlled by an "analog" out?

I have a 16x2 character LCD from sparkfun (https://www.sparkfun.com/datasheets/LCD/GDM1602K.pdf). I'd like to use the liquidcrystal library to drive it. I've never had any problems in the past with this display or the library, but now I'm trying to use pin A3 to drive RS. The display isn't showing anything (backlight works, but other than that nothing else). I looked with my scope at RS and I see that it's always low. I looked at the liquidcrystal.cpp file and I see that RS should be toggled. Any reason that using A3 is a no-no? All the examples use 12, but I can't see why this makes any difference. Thanks.

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Any reason that using A3 is a no-no?

No. However, you might like to call it pin 17.