Maxlength for multiline textbox

<script type="text/javascript">
 function maxLength(field,maxChars)
 {
       if(field.value.length >= maxChars) {
           event.returnValue = false;
           alert("more than " + maxChars + " chars");
           field.value = field.value.substring(0, maxChars);
          return false;
       }


 }
 </script>

<asp:TextBox ID="TextBox1" runat="server" TextMode="MultiLine" onkeydown="return maxLength(this,'50');"></asp:TextBox>