".input-group-addon" results in thick border when used between two inputs

Created by: ghost

Take this piece of code, for example. The width of the right and left border on the input-group-addon are two pixels, while they should be one.

 <div class="input-group">
  <input type="text" class="form-control" >
  <span class="input-group-addon">Hello</span>
  <input type="text" class="form-control" >
</div>