incompatible character encodings: UTF-8 and ASCII-8BIT in render action

ActionView::Template::Error (incompatible character encodings: UTF-8 and ASCII-8BIT): app/controllers/posts_controller.rb:27:in `new'

  # GET /posts/new  def new    if params[:post]      @post = Post.new(post_params).dup      if @post.valid?        render :action => "confirm"      else        format.html { render action: 'new' }        format.json { render json: @post.errors, status: :unprocessable_entity }      end    else      @post = Post.new      @document = Document.new      @documents = @post.documents.all      @document = @post.documents.build    end

I don't know why it is happening.