diff --git a/llvm/lib/Support/Windows/Path.inc b/llvm/lib/Support/Windows/Path.inc
--- a/llvm/lib/Support/Windows/Path.inc
+++ b/llvm/lib/Support/Windows/Path.inc
@@ -352,13 +352,13 @@
 static std::error_code realPathFromHandle(HANDLE H,
                                           SmallVectorImpl<wchar_t> &Buffer) {
   DWORD CountChars = ::GetFinalPathNameByHandleW(
-      H, Buffer.begin(), Buffer.capacity() - 1, FILE_NAME_NORMALIZED);
-  if (CountChars > Buffer.capacity()) {
+      H, Buffer.begin(), Buffer.capacity(), FILE_NAME_NORMALIZED);
+  if (CountChars && CountChars >= Buffer.capacity()) {
     // The buffer wasn't big enough, try again.  In this case the return value
     // *does* indicate the size of the null terminator.
     Buffer.reserve(CountChars);
     CountChars = ::GetFinalPathNameByHandleW(
-        H, Buffer.data(), Buffer.capacity() - 1, FILE_NAME_NORMALIZED);
+        H, Buffer.data(), Buffer.capacity(), FILE_NAME_NORMALIZED);
   }
   if (CountChars == 0)
     return mapWindowsError(GetLastError());