title: Can't execute jupyter notebook in terminal: "execution_count": null
body: <p>I have a test jupyter notebook &quot;Untitled.ipynb&quot; which just consists of one cell with <code>print(&quot;Hello&quot;)</code>.
  As a text file this notebook looks like this:</p>
  <pre><code>{
   &quot;cells&quot;: [
    {
     &quot;cell_type&quot;: &quot;code&quot;,
     &quot;execution_count&quot;: null,
     &quot;metadata&quot;: {},
     &quot;outputs&quot;: [],
     &quot;source&quot;: [
      &quot;print(\&quot;Hello\&quot;)&quot;
     ]
    }
   ],
   &quot;metadata&quot;: {
    &quot;kernelspec&quot;: {
     &quot;display_name&quot;: &quot;test-kernel&quot;,
     &quot;language&quot;: &quot;python&quot;,
     &quot;name&quot;: &quot;test-kernel&quot;
    },
    &quot;language_info&quot;: {
     &quot;codemirror_mode&quot;: {
      &quot;name&quot;: &quot;ipython&quot;,
      &quot;version&quot;: 3
     },
     &quot;file_extension&quot;: &quot;.py&quot;,
     &quot;mimetype&quot;: &quot;text/x-python&quot;,
     &quot;name&quot;: &quot;python&quot;,
     &quot;nbconvert_exporter&quot;: &quot;python&quot;,
     &quot;pygments_lexer&quot;: &quot;ipython3&quot;,
     &quot;version&quot;: &quot;3.7.3&quot;
    }
   },
   &quot;nbformat&quot;: 4,
   &quot;nbformat_minor&quot;: 4
  }
  </code></pre>
  <p>I can perfectly execute it in the .ipynb file, but when I want to run it in the terminal with <code>ipython Untitled.ipynb</code> it returns</p>
  <pre><code>(env) ...$ ipython Untitled.ipynb 
  ---------------------------------------------------------------------------
  NameError                                 Traceback (most recent call last)
  ~/.../Untitled.ipynb in &lt;module&gt;
        3   {
        4    &quot;cell_type&quot;: &quot;code&quot;,
  ----&gt; 5    &quot;execution_count&quot;: null,
        6    &quot;metadata&quot;: {},
        7    &quot;outputs&quot;: [],
  
  NameError: name 'null' is not defined
  </code></pre>
  <p>Based on my google results I manually changed &quot;execution_count&quot; to an integer (e.g. 0, 1) and <code>None</code>.   In both cases the error disappears but the terminal doesn't print anything. It just terminates. The weird thing is that it worked a few days ago...The system runs with Linux Ubuntu 18.04, the virtualenv is activated and <code>pip list</code> contains normally everything you need to run an ipython kernel (see below).</p>
  <pre><code>ipykernel              5.3.4
  ipython                7.2.0
  ipython-genutils       0.2.0
  jupyter-client         6.1.7
  jupyter-core           4.6.3
  </code></pre>
  <p>Does anyone have any idea how to solve this or any hints what I could try?</p>