.invalid-feedback doesn't show for .input-group with an invalid control

Created by: simmerz

Example:

<div class="form-group col-md-4">
  <label class="form-control-label is-invalid" for="valuation_value">Estimated Value</label>
  <div class="input-group is-invalid">
    <span class="input-group-addon">£</span>
    <input class="form-control is-invalid" min="0" type="number" value="" name="valuation[value]">
  </div>
  <div class="invalid-feedback">can't be blank, is not a number</div>
</div>

Results in:

I would expect to see can't be blank, is not a number under the input-group.