title: Output error if there is value under NaN header in Excel file
body: <p>I have inputed Excel table, that look like this:</p>
  <pre><code>head_1 head_2 head_3
  val_1  val_2  val_3
  val_4  val_5  val_6  val_7
  </code></pre>
  <p>I need to output error, because under the NaN header there is value(val_7), but i have no idea how to implement it</p>