Confusing error message

I had an idea to call a variable `up` and here is what I got:
```ruby
[8] pry(main)> up=Class.new
=> #<Class:0x000000118f6130>
[9] pry(main)> up = Class.new
Error: No frame that matches = found!
```
It is a bit confusing message, I did not even knew I was using `pry-stack_explorer` - it come with the project.