diff --git a/lib/logger.js b/lib/logger.js
index 9a5652a7f..7668a691c 100644
--- a/lib/logger.js
+++ b/lib/logger.js
@@ -94,7 +94,7 @@ class Logger {
 
   _parseLevel (level) {
     if (typeof level === 'number') return level
-    level = level.toUpperCase()
+    if (typeof level === 'string') level = level.toUpperCase()
     if (level === 'OFF' || level === '0') return 0
     if (level === 'ERR' || level === 'ERROR' || level === '1') return 1
     if (level === 'INF' || level === 'INFO' || level === '2') return 2