Fix nyc misconfiguration

.babelrc

@@ -1,3 +1,9 @@
 {
-  "presets": ["env"]
+  "presets": ["env"],
+  "env": {
+    "test": {
+      "plugins": [ "istanbul" ],
+      "presets": [["env", { "targets": { "node": "current" } }]]
+    }
+  }
 }

.nycrc

@@ -0,0 +1,4 @@
+{
+  "sourceMap": false,
+  "instrument": false
+}

package.json

@@ -29,6 +29,7 @@
   "devDependencies": {
     "ava": "^0.25.0",
     "babel-cli": "^6.26.0",
+    "babel-plugin-istanbul": "^4.1.6",
     "babel-preset-env": "^1.7.0",
     "babel-register": "^6.26.0",
     "coveralls": "^3.0.2",

src/bowser.js

@@ -25,8 +25,8 @@ class Bowser {
    * bowser.getBrowser()
    */
   constructor(UA, skipParsing=false) {
-    if (!UA) {
+    if (typeof UA !== 'string') {
-      throw new Error('UserAgent is not defined');
+      throw new Error('UserAgent should be a string');
     }
     return new Parser(UA, skipParsing);
   }