diff --git a/index.js b/index.js
index 3008b16..8a1b3d7 100644
--- a/index.js
+++ b/index.js
@@ -68,8 +68,14 @@ const LocalWebDriverBase = function(baseBrowserDecorator, args, logger) {
 
   log.debug('config:', JSON.stringify(config));
 
-  const extraSpecs =
-      _.merge(this.constructor.EXTRA_WEBDRIVER_SPECS, args.config);
+  const extraSpecs = _.mergeWith(
+      this.constructor.EXTRA_WEBDRIVER_SPECS,
+      args.config,
+      (objValue, srcValue) => {
+        if (Array.isArray(objValue)) {
+          return objValue.concat(srcValue);
+        }
+      });
   log.debug('extraSpecs:', extraSpecs);
 
   // These names ("browser" and "spec") are needed for compatibility with