Confusing error message

[mpapis]

I had an idea to call a variable up and here is what I got:

[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.