diff --git a/Lib/posixpath.py b/Lib/posixpath.py
index 16b8d9091c5a84..000fe70e41807a 100644
--- a/Lib/posixpath.py
+++ b/Lib/posixpath.py
@@ -558,7 +558,7 @@ def commonpath(paths):
     try:
         split_paths = [path.split(sep) for path in paths]
 
-        if len({p.startswith(sep) for p in paths}) != 1:
+        if len({p[:1] == sep for p in paths}) != 1:
             raise ValueError("Can't mix absolute and relative paths")
 
         split_paths = [[c for c in s if c and c != curdir] for s in split_paths]
@@ -570,7 +570,7 @@ def commonpath(paths):
                 common = s1[:i]
                 break
 
-        prefix = sep if paths[0].startswith(sep) else sep[:0]
+        prefix = sep if paths[0][:1] == sep else sep[:0]
         return prefix + sep.join(common)
     except (TypeError, AttributeError):
         genericpath._check_arg_types('commonpath', *paths)