Shader error when using UV sets above 4

When trying to use UV sets above 4, I'm getting the following error(s):
Shader error in 'TestShader': invalid subscript 'texcoord5' at line 237 (on d3d11)
Shader warning in 'TestShader': Output value 'vert' is not completely initialized at line 169 (on d3d11)
Any ideas?
Shader error in 'TestShader': invalid subscript 'texcoord5' at line 237 (on d3d11)
Shader warning in 'TestShader': Output value 'vert' is not completely initialized at line 169 (on d3d11)
Any ideas?