diff --git a/html/force.js b/html/force.js
index 8e8b696..36a2ce5 100644
--- a/html/force.js
+++ b/html/force.js
@@ -45,6 +45,9 @@ cp.append("input")
 
 function show_node(mac) {
   d3.selectAll("#chart .node").classed("marked", false);
+  if (mac.length == 0)
+    return;
+
   d3.selectAll("#chart .node").each( function(d) {
     if (d.id == mac) 
       d3.select(this).classed("marked", true);