diff --git a/llvm/utils/lit/lit/util.py b/llvm/utils/lit/lit/util.py
--- a/llvm/utils/lit/lit/util.py
+++ b/llvm/utils/lit/lit/util.py
@@ -151,6 +151,10 @@
             from ctypes import GetLastError, WinError
 
             path = os.path.abspath(path)
+            # Make sure that the path uses backslashes here, in case
+            # python would have happened to use forward slashes, as the
+            # NT path format only supports backslashes.
+            path = path.replace('/', '\\')
             NTPath = to_unicode(r'\\?\%s' % path)
             if not windll.kernel32.CreateDirectoryW(NTPath, None):
                 raise WinError(GetLastError())