There was a discussion about this already (Kotlin JS - annotations processor - #10 by jdemeulenaere). We did not find a solution to check for nullability on generic parameters sadly. The problem is that the annotation processor only gives access to the java type descriptions, which do not contain any information about nullability. This should be solved once we get access to the Kotlin type information. I think JetBrains is planning an creating a better way of doing annotation processing which will give us access to those information, but as far as I remember they are not yet actively working on it (at least they did not tell us about it) and there is definitely no release date.