diff --git a/lib/manager/npm/post-update/index.js b/lib/manager/npm/post-update/index.js
index c7ffd9fa2a742f2948b9859200fb1675481e5bd5..0932df833de73e28b5f1197cb495a73f7e03bea9 100644
--- a/lib/manager/npm/post-update/index.js
+++ b/lib/manager/npm/post-update/index.js
@@ -505,7 +505,7 @@ async function getAdditionalFiles(config, packageFiles) {
                   const localModified = upath.join(config.localDir, f);
                   updatedArtifacts.push({
                     name: f,
-                    contents: await fs.readFile(localModified, 'utf8'),
+                    contents: await fs.readFile(localModified),
                   });
                 }
               }