diff --git a/lib/core/mergeConfig.js b/lib/core/mergeConfig.js
index 2aee6b895c..671e3345f6 100644
--- a/lib/core/mergeConfig.js
+++ b/lib/core/mergeConfig.js
@@ -95,7 +95,7 @@ export default function mergeConfig(config1, config2) {
     headers: (a, b) => mergeDeepProperties(headersToObject(a), headersToObject(b), true)
   };
 
-  utils.forEach(Object.keys(config1).concat(Object.keys(config2)), function computeConfigValue(prop) {
+  utils.forEach(Object.keys(Object.assign({}, config1, config2)), function computeConfigValue(prop) {
     const merge = mergeMap[prop] || mergeDeepProperties;
     const configValue = merge(config1[prop], config2[prop], prop);
     (utils.isUndefined(configValue) && merge !== mergeDirectKeys) || (config[prop] = configValue);