diff --git a/Misc/NEWS.d/next/Windows/2018-04-20-03-24-07.bpo-33316.9IiJ8J.rst b/Misc/NEWS.d/next/Windows/2018-04-20-03-24-07.bpo-33316.9IiJ8J.rst
new file mode 100644
index 00000000000..55176791a90
--- /dev/null
+++ b/Misc/NEWS.d/next/Windows/2018-04-20-03-24-07.bpo-33316.9IiJ8J.rst
@@ -0,0 +1 @@
+PyThread_release_lock always fails
diff --git a/Python/thread_nt.h b/Python/thread_nt.h
index 21ef5556a6b..fdb192b7d77 100644
--- a/Python/thread_nt.h
+++ b/Python/thread_nt.h
@@ -104,8 +104,9 @@ LeaveNonRecursiveMutex(PNRMUTEX mutex)
     if (PyMUTEX_LOCK(&mutex->cs))
         return FALSE;
     mutex->locked = 0;
-    result = PyCOND_SIGNAL(&mutex->cv);
-    result &= PyMUTEX_UNLOCK(&mutex->cs);
+    /* condvar APIs return 0 on success. We need to return TRUE on success. */
+    result = !PyCOND_SIGNAL(&mutex->cv);
+    PyMUTEX_UNLOCK(&mutex->cs);
     return result;
 }