diff --git a/lib/core/mergeConfig.js b/lib/core/mergeConfig.js
index bbbf96d4ff..e4600e5781 100644
--- a/lib/core/mergeConfig.js
+++ b/lib/core/mergeConfig.js
@@ -3,7 +3,7 @@
 import utils from '../utils.js';
 import AxiosHeaders from "./AxiosHeaders.js";
 
-const headersToObject = (thing) => thing instanceof AxiosHeaders ? thing.toJSON() : thing;
+const headersToObject = (thing) => thing instanceof AxiosHeaders ? { ...thing } : thing;
 
 /**
  * Config-specific merge-function which creates a new config-object