Index: llvm/trunk/lib/Support/Windows/Process.inc
===================================================================
--- llvm/trunk/lib/Support/Windows/Process.inc
+++ llvm/trunk/lib/Support/Windows/Process.inc
@@ -129,9 +129,10 @@
   size_t Size = MAX_PATH;
   do {
     Buf.reserve(Size);
+    SetLastError(NO_ERROR);
     Size =
-        GetEnvironmentVariableW(NameUTF16.data(), Buf.data(), Buf.capacity());
-    if (Size == 0)
+      GetEnvironmentVariableW(NameUTF16.data(), Buf.data(), Buf.capacity());
+    if (Size == 0 && GetLastError() == ERROR_ENVVAR_NOT_FOUND)
       return None;
 
     // Try again with larger buffer.
Index: llvm/trunk/unittests/Support/ProcessTest.cpp
===================================================================
--- llvm/trunk/unittests/Support/ProcessTest.cpp
+++ llvm/trunk/unittests/Support/ProcessTest.cpp
@@ -42,10 +42,18 @@
   Optional<std::string> val(
       Process::GetEnv("__LLVM_TEST_ENVIRON_NO_SUCH_VAR__"));
   EXPECT_FALSE(val.hasValue());
-}
+} 
 #endif
 
 #ifdef LLVM_ON_WIN32
+
+TEST(ProcessTest, EmptyVal) {
+  SetEnvironmentVariableA("__LLVM_TEST_ENVIRON_VAR__", "");
+  Optional<std::string> val(Process::GetEnv("__LLVM_TEST_ENVIRON_VAR__"));
+  EXPECT_TRUE(val.hasValue());
+  EXPECT_STREQ("", val->c_str());
+}
+
 TEST(ProcessTest, Wchar) {
   SetEnvironmentVariableW(L"__LLVM_TEST_ENVIRON_VAR__", L"abcdefghijklmnopqrs");
   Optional<std::string> val(Process::GetEnv("__LLVM_TEST_ENVIRON_VAR__"));