Can't access image_tag from helper module

I would like to test the following helper module function:

module UploadsHelper

  def custom_img_tag(upload, width, height, id)
     if width > Upload::MAX_CROP_WIDTH
       image_tag(upload.photo.url(:original), :id => "box", :width => Upload::MAX_CROP_WIDTH, :height => (height*Upload::MAX_CROP_WIDTH/width).to_i)
     else
       image_tag(upload.photo.url(:original), :id => "box")
     end
   end

end

However when I run the following test:

describe UploadsController do
  include UploadsHelper
    describe "custom_img_tag(upload, width, height, id)" do
           before(:each) do
             @upload = Factory(:upload)
             geo = Paperclip::Geometry.from_file(@upload.photo.to_file(:original))
             @width   = geo.width
             @开发者_StackOverflow中文版height  = geo.height
           end

       it "should return the original image tag for an image that is not wider than  MAX_CROP_WIDTH" do
         #custom_img_tag(@upload,@width, @heigth, "cropbox" ).should == '<img id="cropbox" width="500" height="375" src="/system/photos/10/original/avatar.jpg?1311044917" alt="Avatar" style="display: none;">'
       end
     end

I get the following error:

Failure/Error: custom_img_tag(@upload,@width, @heigth, "cropbox" ).should == '<img id="cropbox" width="500" height="375" src="/system/photos/10/original/avatar.jpg?1311044917" alt="Avatar" style="display: none;">'
     NoMethodError:
       You have a nil object when you didn't expect it!

Why do I get this error and how can I test this method?

Update: I added the following to the spec test file:

include ActionView::Helpers 

Which produces the following error:

NameError:
       undefined local variable or method `config' for #<RSpec

How can I get rid of this error and what is the cause?

Thanks for any assistance.

I also got hit with the error working with Rails 3.1 RC

NameError:
   undefined local variable or method `config'

Some Rails source tracing and I discovered the missing include ActionView::AssetPaths.

include ActionView::AssetPaths
include ActionView::Helpers::AssetTagHelper

Well, I don't know why this would be, but my guess is that for some reason ActionView::Helpers must not be loaded in this spec. Try including ActionView::Helpers and see if that fixes it... The issue (from what you're reporting) is that when your custom_img_tag method gets called it isn't able to call image_tag for some reason.