Your code seems alright to me! My guess for the error is that it seems like it might be interacting oddly with another shader you have in your program. Otherwise, were you able to recreate the error yourself? Does it only occur on Android, or on a development or PC build, too?
I did(do?) have a gradient that was slightly edited from tom's example gradient shader code on the documentation page -- though the android version doesn't/shouldn't be using it at all at this point....
I think it's only on android, though the phone I used to test was fine + the PC builds arent having an issue-- At this point I think I might have to wait for the person who got this to get back to me (though I'll update the android dl to have absolutely nothing else shader-wise.)
Thank you for helping me narrow it down! ><