diff --git a/src/path.ts b/src/path.ts
index 48f5e1e..27e1327 100644
--- a/src/path.ts
+++ b/src/path.ts
@@ -218,7 +218,7 @@ export const format: typeof path.format = function (p) {
 // basename
 export const basename: typeof path.basename = function (p, ext) {
   const lastSegment = normalizeWindowsPath(p).split('/').pop()
-  return lastSegment.endsWith(ext) ? lastSegment.slice(0, -ext.length) : lastSegment
+  return ext && lastSegment.endsWith(ext) ? lastSegment.slice(0, -ext.length) : lastSegment
 }
 
 // parse
diff --git a/test/index.spec.ts b/test/index.spec.ts
index 5a5b3cc..64370af 100644
--- a/test/index.spec.ts
+++ b/test/index.spec.ts
@@ -62,12 +62,14 @@ runTest('basename', basename, [
   ['/temp/myfile.html', 'myfile.html'],
   ['./myfile.html', 'myfile.html'],
   ['./myfile.html', '.html', 'myfile'],
+  ['./undefined', undefined, 'undefined'],
 
   // Windows
   ['C:\\temp\\myfile.html', 'myfile.html'],
   ['\\temp\\myfile.html', 'myfile.html'],
   ['.\\myfile.html', 'myfile.html'],
-  ['.\\myfile.html', '.html', 'myfile']
+  ['.\\myfile.html', '.html', 'myfile'],
+  ['.\\undefined', undefined, 'undefined']
 ])
 
 runTest('dirname', dirname, {