This parameter has to be of type: [UNREGISTERED]

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

 Script error: OnTriggerEnter2D
 This message parameter has to be of type: [UNREGISTERED]
 The message will be ignored.
  
 (Filename: /Users/builduser/buildslave/unity/build/Runtime/Mono/MonoScriptCache.cpp Line: 248)
 Script error: OnTriggerExit2D
 This message parameter has to be of type: [UNREGISTERED]
 The message will be ignored.
  
 (Filename: /Users/builduser/buildslave/unity/build/Runtime/Mono/MonoScriptCache.cpp Line: 248)
 Script error: OnTriggerStay2D
 This message parameter has to be of type: [UNREGISTERED]
 The message will be ignored.

My methods for this are defined like this:

     private void OnTriggerEnter2D(Collider2D other)
     {
         // ...
     }

     private void OnTriggerExit2D(Collider2D other)
     {
         // ...
     }

     private void OnTriggerStay2D(Collider2D other)
     {
         // ...
     }

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?