ruby on rails 4 - NoMethodError (undefined method `[]' for nil:NilClass) Post Attachments issue -

i getting error , not sure why. post still saved database, though throws , error. has post_attachments, not sure how fix it. appreciated!

here postcontroller create method error:

def create     @trail = trail.find(params[:trail_id])     @post = current_user.posts.build(params.require(:post).permit(:body, post_attachments_attributes: [:id, :post_id, :gallery]))     @post.trail = @trail     @new_post = post.new     authorize @post  if @post.save   params[:post_attachments]['gallery'].each |a|       @post_attachment = @post.post_attachments.create!(:gallery => a, :post_id => @post.id)   end   flash[:notice] = "post saved!"   redirect_to @post.trail else   flash[:error] = "error creating post. please try again."   render :new end 

end

the error occurs on line params[:post_attachments]['gallery'].each |a|