title: "False" is being inserted to column instead of stated value when using fillna in pandas, python
body: <p>I'm trying to fill in values in one column with a specific value based on what value is in another column. E.g I want small to have one value in and medium nulls to have another value.</p>
  <pre><code>| Size | Number |
  |:-----|:-------|
  |Small | 34 |
  |Small |  |
  |Medium| 45|
  |Large| 72|
  |Medium|  |
  |Small|  |
  |Large| 89|
  |Small| 45|
  |Medium| 32|
  |Medium|  |
  </code></pre>
  <p>I'm using</p>
  <pre><code>df['Number'].fillna(df.groupby(df['Size']=='Small') == 10, inplace = True)
  </code></pre>
  <p>but this resulting in:</p>
  <pre><code>| Size | Number |
  |:-----|:-------|
  |Small | 34 |
  |Small | FALSE |
  |Medium| 45|
  |Large| 72|
  |Medium|  |
  |Small|  FALSE|
  |Large| 89|
  |Small| 45|
  |Medium| 32|
  |Medium|  |
  </code></pre>
  <p>I'm expecting '10' to be added where False has been put.
  I've been stuck on this for ages. Can anyone tell me why it isn't inserting the value? Its finding the right rows to fill in but just not adding the value I want.</p>
  <p>Please note that I'm working with a much larger database loaded from file and this is just an example df for my issue. Thank you.</p>