diff --git a/lib/core/mergeConfig.js b/lib/core/mergeConfig.js
index 3b3d610..e8782ab 100644
--- a/lib/core/mergeConfig.js
+++ b/lib/core/mergeConfig.js
@@ -16,7 +16,9 @@ module.exports = function mergeConfig(config1, config2) {
   var config = {};
 
   utils.forEach(['url', 'method', 'params', 'data'], function valueFromInstanceConfig(prop) {
-    config[prop] = config2[prop];
+    if (typeof config2[prop] !== 'undefined') {
+      config[prop] = config2[prop];
+    }
   });
 
   utils.forEach(['headers', 'auth', 'proxy'], function mergeInstanceConfigWithDefaults(prop) {