Type was not serializable and has no concrete serializable subtypes

The internet has really helped to speed up development. I remember when I first started programming and you had to decipher what you had done wrong and wasted hours and sometimes days to figure it out.

Again I came across an error in my GWT RPC application where the message is not exactly clear as to what it is that is not correct.

[ERROR] Type 'a.b.c.client.model.Activity' was not serializable and has no concrete serializable subtypes
[ERROR] Line 30: Failed to resolve 'a.b.c.client.rpc.ActivityService' via deferred binding

Luckily I remember something about a no-argument constructor in my class and that was it. Searching in GOOGLE one also finds the answer immediately.