diff --git a/Lib/posixpath.py b/Lib/posixpath.py
index 14b9270..1be5d29 100644
--- a/Lib/posixpath.py
+++ b/Lib/posixpath.py
@@ -118,14 +118,21 @@
 def commonprefix(m):
     "Given a list of pathnames, returns the longest common leading component"
     if not m: return ''
-    prefix = m[0]
-    for item in m:
+    n = m[:]
+    for i in range(len(n)):
+        n[i] = n[i].split(os.sep)
+        # if os.sep didn't have any effect, try os.altsep
+        if os.altsep and len(n[i]) == 1:
+            n[i] = n[i].split(os.altsep)
+            
+    prefix = n[0]
+    for item in n:
         for i in range(len(prefix)):
             if prefix[:i+1] <> item[:i+1]:
                 prefix = prefix[:i]
                 if i == 0: return ''
                 break
-    return prefix
+    return os.sep.join(prefix)
 
 
 # Get size, mtime, atime of files.