diff --git a/lib/core.js b/lib/core.js
index bd7ed512f..b8399cbce 100644
--- a/lib/core.js
+++ b/lib/core.js
@@ -1042,13 +1042,13 @@
                                     allResults.vars[key] = utils.filterSameObjects(allResults.vars[key]);
 
                                 } else {
-                                    // Merge object mode.
+                                    // Merge object mode. Prevent mutation.
                                     if (newValueOverrides) {
                                         // Extend existing object.
-                                        Object.assign(oldValue, v);
+                                        allResults.vars[key] = Object.assign({}, oldValue, v);
                                     } else {
                                         // Extend with old values on top.
-                                        allResults.vars[key] = Object.assign(v, oldValue);
+                                        allResults.vars[key] = Object.assign({}, v, oldValue);
                                     }
                                 }