Steps to repro:

1. Open attached project and 't1' scene.

Expected result:
No errors thrown.

Actual result:

2 errors:

- Assertion failed: Assertion failed on expression: 'scriptParameterClass != NULL'

and

- Script error: OnTerrainChanged
This message parameter has to be of type: int
The message will be ignored.

are thrown in the console. See attached screenshot.

Regression introduced in Unity 2017.2.0a2.

Notes:
- Reproducible in 2017.2.0b2, 2017.1.0a2;
- Not reproducible in 2017.1.0a1, 2017.1.0f2, 5.6.1p2;
- Reproducible on OSX 10.12.5 and Windows 10.