OnServerClick Only Works When Onclick Not Defined

protected void btnNext_Click(object sender, EventArgs e) { btnNext.InnerHtml = "CLICK";
} <button type="submit" runat="server" onserverclick="btnNext_Click" id="btnNext">Next > </button>

This works fine, but when I add an onclick event:
<button type="submit" runat="server" onserverclick="btnNext_Click" onclick="return checkForm();" id="btnNext">Next > </button>

And in the head:
<script type="text/javascript"> function checkForm() {
if (document.getElementById("<%=lstChooseSpec.ClientID %>").value) {
return true; } else {
$.jGrowl("<strong>Warning!</strong><br />Please select an item", { sticky: true });
return false; } } </script>

It submits the form, but doesn't change the buttons text to 'CLICK'. The text only changes to 'CLICK' when the onclick() isn't defined, although the form is still submitting!