I don't understand this build error

I did not use the Nature/SpeedTree shader at all in my project. Neither did I use any shaders from the Nature category, or any custom shaders. However whenever I build my project, I get the 2 error messages: (1) "Shader error in 'Nature/SpeedTree': Did not find shader kernel 'frag_surf' to compile (on d3d9)" (2) "Shader error in 'Nature/SpeedTree': Did not find shader kernel 'frag_surf' to compile (on d3d11)" The project runs when I run it directly from inside the Unity editor, like when I hit the play button at the top of the screen. However I am unable to build the project.