diff --git a/lib/levels.js b/lib/levels.js
index 1b4bacd47..7eb66c42a 100644
--- a/lib/levels.js
+++ b/lib/levels.js
@@ -117,7 +117,7 @@ function assertDefaultLevelFound (defaultLevel, customLevels, useOnlyCustomLevel
   if (typeof defaultLevel === 'number') {
     const values = [].concat(
       Object.keys(customLevels || {}).map(key => customLevels[key]),
-      useOnlyCustomLevels ? [] : Object.keys(nums),
+      useOnlyCustomLevels ? [] : Object.keys(nums).map(level => +level),
       Infinity
     )
     if (!values.includes(defaultLevel)) {
diff --git a/test/levels.test.js b/test/levels.test.js
index 064be32f9..f8ee7ae6f 100644
--- a/test/levels.test.js
+++ b/test/levels.test.js
@@ -381,3 +381,9 @@ test('throws when creating a default value that does not exist in logger levels'
     is(message, `default level:info must be included in custom levels`)
   }
 })
+
+test('passes when creating a default value that exists in logger levels', async ({ is, throws }) => {
+  pino({
+    level: 30
+  })
+})