diff --git a/Lib/distutils/command/build_py.py b/Lib/distutils/command/build_py.py
index 527e81d9ae4..97d094b1b29 100644
--- a/Lib/distutils/command/build_py.py
+++ b/Lib/distutils/command/build_py.py
@@ -12,6 +12,7 @@ from glob import glob
 
 from distutils.core import Command
 from distutils.errors import *
+from distutils.util import convert_path
 
 
 class build_py (Command):
@@ -50,7 +51,10 @@ class build_py (Command):
         # options -- list of packages and list of modules.
         self.packages = self.distribution.packages
         self.py_modules = self.distribution.py_modules
-        self.package_dir = self.distribution.package_dir
+        self.package_dir = {}
+        if self.distribution.package_dir:
+            for name, path in self.distribution.package_dir.items():
+                self.package_dir[name] = convert_path(path)
 
         # Ick, copied straight from install_lib.py (fancy_getopt needs a
         # type system!  Hell, *everything* needs a type system!!!)