lg vs sm in input groups of bootstrap

I am unable to get the following working:

<div class="input-group-sm">
                        <input type="text" class="form-control">
                        <div class="input-group-sm">
                            <button class="btn btn-default btn-sm" type="button">
                                <i class="glyphicon glyphicon-remove"></i>
                            </button>
                            <button class="btn btn-default btn-sm" type="button">
                                <i class="glyphicon glyphicon-search"></i>
                            </button>
                        </div>
                    </div>

I wanted to have the textbox and both buttons to be on the same line, tied to each other. The above code makes the buttons to be on second line. Can anyone help me on this.

It works well if replace "sm" with "lg" everywhere.