diff --git a/lib/index.js b/lib/index.js
index ec731881..8142f626 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -1,6 +1,7 @@
 import ironStore from "iron-store";
 import cookie from "cookie";
 import createDebug from "debug";
+import flat from "array.prototype.flat";
 
 const debug = createDebug("next-iron-session");
 
@@ -67,7 +68,7 @@ export async function applySession(
           `next-iron-session: Cookie length is too big ${cookieValue.length}, browsers will refuse it`,
         );
       }
-      const existingSetCookie = [res.getHeader("set-cookie") || []].flat();
+      const existingSetCookie = flat([res.getHeader("set-cookie") || []]);
       res.setHeader("set-cookie", [...existingSetCookie, cookieValue]);
       return cookieValue;
     },
@@ -77,7 +78,7 @@ export async function applySession(
         ...cookieOptions,
         maxAge: 0,
       });
-      const existingSetCookie = [res.getHeader("set-cookie") || []].flat();
+      const existingSetCookie = flat([res.getHeader("set-cookie") || []]);
       res.setHeader("set-cookie", [...existingSetCookie, cookieValue]);
     },
   };
diff --git a/package.json b/package.json
index 2ac220da..17b8bdd5 100644
--- a/package.json
+++ b/package.json
@@ -32,9 +32,7 @@
         {
           "targets": {
             "node": "10"
-          },
-          "useBuiltIns": "usage",
-          "corejs": 3
+          }
         }
       ]
     ]
@@ -81,6 +79,7 @@
     ]
   },
   "dependencies": {
+    "array.prototype.flat": "^1.2.3",
     "clone": "^2.1.2",
     "cookie": "^0.4.1",
     "debug": "^4.1.1",