col-md-3 lines are not proper code example

Example: col-md-3 lines are not proper

<div id="mysection">
    <div class="container">
        <div class="row">
            <div class="col-md-3 equal-column"></div>
            <div class="col-md-3 equal-column"></div>
            <div class="col-md-3 equal-column"></div>
            <div class="col-md-3 equal-column"></div>
            <div class="col-md-3 equal-column"></div>
            <div class="col-md-3 equal-column"></div>
            <div class="col-md-3 equal-column"></div>
            <div class="col-md-3 equal-column"></div>

        </div>
    </div>
</div>

Add CSS:
.equal-column{min-height:300px;}