Why does computed throw an error when changing a property on an event?

Question

Vue.js

Greetings.
I'm learning Vue.
I worked with computed and came across an error (example):
jsbin.com/buqebajape/edit?html,js,console,output
When loading, everything works, but when I click on the span, an error occurs. Although if you change the property through the console, then everything works out.
Why?

Answer 1

When the reverse method is called, it makes this.message a number, and the number does not have a split method, which causes an error. Look at the browser console:

TypeError: this.message.split is not a function
    at pt.reversedMessage (milihexife.js:14:37)
...