This parameter has to be of type: [UNREGISTERED]

I'm getting the following errors when building for iOS:

Code (CSharp):

1. Script error: OnTriggerEnter2D
2. This message parameter has to be of type: [UNREGISTERED]
3. The message will be ignored.
4. (Filename: /Users/builduser/buildslave/unity/build/Runtime/Mono/MonoScriptCache.cpp Line: 248)
5.  
6. Script error: OnTriggerExit2D
7. This message parameter has to be of type: [UNREGISTERED]
8. The message will be ignored.
9. (Filename: /Users/builduser/buildslave/unity/build/Runtime/Mono/MonoScriptCache.cpp Line: 248)
10.  
11. Script error: OnTriggerStay2D
12. This message parameter has to be of type: [UNREGISTERED]
13. The message will be ignored.

My methods for this are defined like this:

Code (CSharp):

1.      private void OnTriggerEnter2D(Collider2D other)
2.      {
3.          // ...
4.      }
5.      private void OnTriggerExit2D(Collider2D other)
6.      {
7.          // ...
8.      }
9.      private void OnTriggerStay2D(Collider2D other)
10.      {
11.          // ...
12.      }

I had seen these errors before when the method signature was wrong (e.g. wrong parameter type), but never [UNREGISTERED]. What am I doing wrong?

Note: Collider2D refers to UnityEngine.Collider2D, not another custom type.

 

At the IL level, after compilation, how I qualified the type in C# initially doesn't matter.
My note was just to indicate that indeed, there wasn't for example another colliding type called Collider2D.

 

Any other idea @JeffDUnity3D?

 

I have a similar error. But the project code does not contain the method in the message.