fill-paragraph does not work sometimes in Emacs IPython Notebook

ein_fill_paragraph.ipynb

{
 "metadata": {
  "name": "ein_fill_paragraph"
 },
 "nbformat": 3,
 "nbformat_minor": 0,
 "worksheets": [
  {
   "cells": [
    {
     "cell_type": "markdown",
     "source": [
      "This text cell is very very very very very very very very very very very very very very very very very very very very long and fill-paragraph will return error message"
     ]
    },
    {
     "cell_type": "markdown",
     "source": [
      "This line is ok\n",
      "\n",
      "And this line very very very very very very very very very very very very very very very very very very very very very very long, but fill-paragraph work"
     ]
    }
   ]
  }
 ]
}