title: NA values are not recognized properly using dplyr
body: <p>I have a dataset with two columns, in one of them are missing values.
  I load it using</p>
  <pre><code>data &lt;- read_excel(&quot;file.xlsx&quot;) %&gt;%
    select(&quot;ID&quot;, &quot;Value&quot;)
  </code></pre>
  <p>The tibble looks like that</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>ID</th>
  <th>Value</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>1</td>
  <td>2</td>
  </tr>
  <tr>
  <td>NA</td>
  <td>4</td>
  </tr>
  <tr>
  <td>32</td>
  <td>1</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>The NAs are recognized as such.
  However, I use</p>
  <pre><code>data[&quot;ID&quot;==&quot;NA&quot;] &lt;- NA
  </code></pre>
  <p>to ensure that this is not the problem (<a href="https://stackoverflow.com/questions/30381152/r-is-na-does-not-pick-up-na-value/30381226">R: is.na() does not pick up NA value</a>).</p>
  <p>When I try to filter:</p>
  <pre><code>data %&gt;%
  filter(!is.na(ID))
  </code></pre>
  <p>the whole tibble stays the same, and no row is deleted.
  So I try</p>
  <pre><code>data %&gt;%
  mutate(
  isna &lt;- is.na(ID)
  )
  </code></pre>
  <p>and all isna are FALSE.</p>
  <p>Why doesn't recognize dplyr the NAs?</p>
  <p>I am grateful for every help!</p>