diff --git a/test/lib/next-modes/base.ts b/test/lib/next-modes/base.ts
index f6d2d930226..2e9b7432f9c 100644
--- a/test/lib/next-modes/base.ts
+++ b/test/lib/next-modes/base.ts
@@ -178,10 +178,14 @@ export class NextInstance {
       }
     }
 
-    const nextConfigFile = Object.keys(this.files).find((file) =>
+    let nextConfigFile = Object.keys(this.files).find((file) =>
       file.startsWith('next.config.')
     )
 
+    if (await fs.pathExists(path.join(this.testDir, 'next.config.js'))) {
+      nextConfigFile = 'next.config.js'
+    }
+
     if (nextConfigFile && this.nextConfig) {
       throw new Error(
         `nextConfig provided on "createNext()" and as a file "${nextConfigFile}", use one or the other to continue`