diff --git a/scm.py b/scm.py
index 88016dccd..8708c19b0 100644
--- a/scm.py
+++ b/scm.py
@@ -8,6 +8,7 @@ import cStringIO
 import glob
 import logging
 import os
+import platform
 import re
 import sys
 import tempfile
@@ -258,6 +259,9 @@ class GIT(object):
   def GetOldContents(cwd, filename, branch=None):
     if not branch:
       branch = GIT.GetUpstreamBranch(cwd)
+    if platform.system() == 'Windows':
+      # git show <sha>:<path> wants a posix path.
+      filename = filename.replace('\\', '/')
     command = ['show', '%s:%s' % (branch, filename)]
     try:
       return GIT.Capture(command, cwd=cwd, strip_out=False)
diff --git a/tests/scm_unittest.py b/tests/scm_unittest.py
index 400ca18c1..8ee3d811a 100755
--- a/tests/scm_unittest.py
+++ b/tests/scm_unittest.py
@@ -58,6 +58,7 @@ class RootTestCase(BaseSCMTestCase):
         'logging',
         'only_int',
         'os',
+        'platform',
         're',
         'subprocess2',
         'sys',