diff --git a/src/util/object.ts b/src/util/object.ts
index 2cfcf6d..68abf01 100644
--- a/src/util/object.ts
+++ b/src/util/object.ts
@@ -11,17 +11,18 @@ export const filterUndefined = (object: any) => {
   return obj;
 };
 
-// deep merge nested objects
-// (arrays are concatenated
+// deep merge nested objects - returns a new object
+// (arrays are concatenated)
 export const mergeDeep = (target: any, source: any = {}) => {
+  let output = Object.assign({}, target);
   for (const key in source) {
     if (source[key] instanceof Object && !Array.isArray(source[key]) && key in target) {
-      Object.assign(source[key], mergeDeep(target[key], source[key]));
+      output[key] = mergeDeep(target[key], source[key]);
     } else if (Array.isArray(source[key]) && Array.isArray(target[key])) {
-      target[key] = target[key].concat(source[key]);
+      output[key] = target[key].concat(source[key]);
     } else {
-      target[key] = source[key];
+      output[key] = source[key];
     }
   }
-  return target;
+  return output;
 }