diff --git a/lib/vehicle.js b/lib/vehicle.js
index <HASH>..<HASH> 100644
--- a/lib/vehicle.js
+++ b/lib/vehicle.js
@@ -17,7 +17,12 @@ var createMethod = function(method) {
  * @param {String} token a valid access token
  * @param {string} vid the vehicle's unique identifier
  */
-function Vehicle(vid, token, unitSystem = 'metric') {
+function Vehicle(vid, token, unitSystem) {
+  unitSystem = typeof unitSystem !== 'undefined' ? unitSystem : 'metric';
+  if (['imperial', 'metric'].indexOf(unitSystem) < 0) {
+    throw new TypeError('unit system must be one of: \'imperial\', \'metric\'');
+  }
+
   this.vid = vid;
   this.token = token;
   this.unitSystem = unitSystem;
no default params in node 4, changed