Question 2D shader doesn't have a main texture when using a render texture?

I've been following a couple of tutorials about make a water shader, where a render texture is sampled and applied to the input. In both videos that alone works, but when I try to recreate it, the sprite renderer complains that there is no main texture assigned.
I know the usual workflow of making a 2D texture and setting the reference to _MainTex, but in this case I want to disregard the original anyway and only use the color etc. from the render texture.

Any idea why this isn't working? I've tied a bunch of Unity versions and the error is the same