diff --git a/values.js b/values.js
index 315548f..3d7372e 100644
--- a/values.js
+++ b/values.js
@@ -358,9 +358,11 @@ export const Values = GObject.registerClass({
                 key: sessionKey,
             });
 
-            // calculate speed for this interface
-            let speed = (value - previousValue[1]) / dwell;
+            // calculate speed for this interface - there is nothing to compare
+            // against the very first time an interface is seen
+            let speed = (previousValue)?(value - previousValue[1]) / dwell:0;
             let speedFormatted = this._legible(speed, 'speed', type, key);
+
             output.push({
                 label,
                 value: speedFormatted.text,