diff --git a/lib/workers/branch/yarn.js b/lib/workers/branch/yarn.js
index b9a232e6def5e9d89d403715068912179a2a56fc..8bde6804efa697a99631b6259130f19c71131320 100644
--- a/lib/workers/branch/yarn.js
+++ b/lib/workers/branch/yarn.js
@@ -53,11 +53,11 @@ async function generateLockFile(
     logger.warn(
       {
         err,
-        newPackageJson,
+        newPackageJson: JSON.parse(newPackageJson),
         npmrcContent,
         yarnrcContent,
-        stdout: result.stdout,
-        stderr: result.stderr,
+        stdout: String(result.stdout),
+        stderr: String(result.stderr),
       },
       'Error generating yarn.lock'
     );