invalid property value on border with bootstrap

i get an "invalid property value" with propriety border (outline too), when i would like to add a border-bottom on a div, and i don't know how fix it.

<div class="container">
    <div class="row">
        <div class="maindivs col-md-offset-1 col-sm-7 col-md-7 ">
            <div class="row">
                <div id="navbar">
                    <nav style="border-radius:0px;"class="navbar navbar-default">
                        <div class="container"></div>
                    </nav>
                </div>
                <div style="border-bottom: 1px static black !important;" class="col-lg-12"> 
                    <h2>Coucou</h2>
                </div>
            </div>
        </div>
    </div>
</div>

that's this line : style="border-bottom: 1px static black !important;"

this problem isn't just that this line, because i have same error if i add this style on this div too:

<div class="maindivs col-md-offset-1 col-sm-7 col-md-7 ">

Anyone know how I can fix the problem and add a border on my divs ?

Thank's