carrierwave + mini_magick gems = not an image error

I'm using:

- carrierwave 0.6.0
- rails 3.2.1
- mini_magick 3.4
- ruby 1.9.2p290

And during image uploading I want to resize uploading pic:

(image_uploader.rb)
process :resize_to_fit => [200, 200]

But after image uploading I've got an error:

1 error prohibited this organization from being saved:
Image image Failed to manipulate with MiniMagick, maybe it is not an image? Original Error: MiniMagick::Invalid

PS. If I'm not resizing, image upload works fine.

PS2. I tried to add in config/environment.rb:

require 'rubygems'
require "mini_magick"

but without success (still not working - btw. is it required when using mini_magick as a gem?)