diff --git a/python/mozbuild/mozpack/path.py b/python/mozbuild/mozpack/path.py
index 063d7acb86c7..453bd5353b1c 100644
--- a/python/mozbuild/mozpack/path.py
+++ b/python/mozbuild/mozpack/path.py
@@ -23,9 +23,17 @@ def normsep(path):
     :py:const:`os.sep` is.
     '''
     if os.sep != '/':
-        path = path.replace(os.sep, '/')
+        # Python 2 is happy to do things like byte_string.replace(u'foo',
+        # u'bar'), but not Python 3.
+        if isinstance(path, bytes):
+            path = path.replace(os.sep.encode('ascii'), b'/')
+        else:
+            path = path.replace(os.sep, '/')
     if os.altsep and os.altsep != '/':
-        path = path.replace(os.altsep, '/')
+        if isinstance(path, bytes):
+            path = path.replace(os.altsep.encode('ascii'), b'/')
+        else:
+            path = path.replace(os.altsep, '/')
     return path