diff --git a/lib/workers/branch/lock-files.js b/lib/workers/branch/lock-files.js
index f6f74a95d48281edb1a0d992f09bf60dd70a2886..af518832ab265fb4863ba62869565a6d56f27640 100644
--- a/lib/workers/branch/lock-files.js
+++ b/lib/workers/branch/lock-files.js
@@ -137,6 +137,9 @@ async function writeUpdatedPackageFiles(config) {
     return;
   }
   for (const packageFile of config.updatedPackageFiles) {
+    if (!packageFile.name.endsWith('package.json')) {
+      continue; // eslint-disable-line
+    }
     logger.debug(`Writing ${packageFile.name}`);
     const massagedFile = JSON.parse(packageFile.contents);
     if (massagedFile.name && !massagedFile.name.match(/^[0-9a-z-_]+$/)) {
diff --git a/test/workers/branch/lock-files.spec.js b/test/workers/branch/lock-files.spec.js
index c0ab1c68dce356bf5ab1b0b925737283361abc58..0281c6c54bc3a9c4d3ed0256a443c0333eae22fe 100644
--- a/test/workers/branch/lock-files.spec.js
+++ b/test/workers/branch/lock-files.spec.js
@@ -226,6 +226,16 @@ describe('workers/branch/lock-files', () => {
       await writeUpdatedPackageFiles(config);
       expect(fs.outputFile.mock.calls).toHaveLength(0);
     });
+    it('returns if no updated packageFiles are package.json', async () => {
+      config.updatedPackageFiles = [
+        {
+          name: 'Dockerfile',
+          contents: 'some-contents',
+        },
+      ];
+      await writeUpdatedPackageFiles(config);
+      expect(fs.outputFile.mock.calls).toHaveLength(0);
+    });
     it('writes updated packageFiles', async () => {
       config.updatedPackageFiles = [
         {