diff --git a/ports/win32/qf_port.c b/ports/win32/qf_port.c
index c4e42110..58b76baa 100644
--- a/ports/win32/qf_port.c
+++ b/ports/win32/qf_port.c
@@ -5,7 +5,7 @@
 * @cond
 ******************************************************************************
 * Last updated for version 6.4.0
-* Last updated on  2019-02-07
+* Last updated on  2019-02-12
 *
 *                    Q u a n t u m  L e a P s
 *                    ------------------------
@@ -138,12 +138,11 @@ void QF_setTickRate(uint32_t ticksPerSec, int_t tickPrio) {
 }
 /****************************************************************************/
 void QF_setWin32Prio(QActive *act, int_t win32Prio) {
-    if (act->thread == (HANDLE)0) {  /* thread not created yet? */
-        act->osObject = (void *)win32Prio; /* store the priority for later */
-    }
-    else {
-        SetThreadPriority(act->thread, win32Prio);
-    }
+    HANDLE win32thread = (HANDLE)act->thread;
+
+    /* thread must be already created, see QActive_start_() */
+    Q_REQUIRE_ID(700, win32thread != (HANDLE)0);
+    SetThreadPriority(win32thread, win32Prio);
 }
 
 /* QActive functions =======================================================*/
@@ -152,23 +151,15 @@ void QActive_start_(QActive * const me, uint_fast8_t prio,
                     void *stkSto, uint_fast16_t stkSize,
                     QEvt const *ie)
 {
-    int win32Prio;
-
-    Q_REQUIRE_ID(700, ((uint_fast8_t)0 < prio) /* priority must be in range */
+    Q_REQUIRE_ID(800, ((uint_fast8_t)0 < prio) /* priority must be in range */
                  && (prio <= (uint_fast8_t)QF_MAX_ACTIVE)
                  && (stkSto == (void *)0));    /* statck storage must NOT...
                                                * ... be provided */
-
     me->prio = prio; /* set QF priority of this AO before adding it to QF */
     QF_add_(me);     /* make QF aware of this active object */
 
     QEQueue_init(&me->eQueue, qSto, qLen);
 
-    /* save osObject as integer, in case it contains the Win32 priority */
-    win32Prio = (me->osObject != (void *)0)
-                ? (int)me->osObject
-                : THREAD_PRIORITY_NORMAL;
-
     /* create the Win32 "event" to throttle the AO's event queue */
     me->osObject = CreateEvent(NULL, FALSE, FALSE, NULL);
 
@@ -184,12 +175,7 @@ void QActive_start_(QActive * const me, uint_fast8_t prio,
     * The thread is created with THREAD_PRIORITY_NORMAL
     */
     me->thread = CreateThread(NULL, stkSize, &ao_thread, me, 0, NULL);
-    Q_ASSERT_ID(730, me->thread != (HANDLE)0); /* must succeed */
-
-    /* was the thread priority provided? */
-    if (win32Prio != 0) {
-        SetThreadPriority(me->thread, win32Prio);
-    }
+    Q_ENSURE_ID(830, me->thread != (HANDLE)0); /* must succeed */
 }
 
 /****************************************************************************/
diff --git a/ports/win32/qf_port.h b/ports/win32/qf_port.h
index 30c55d46..6fdd64f4 100644
--- a/ports/win32/qf_port.h
+++ b/ports/win32/qf_port.h
@@ -4,14 +4,14 @@
 * @ingroup ports
 * @cond
 ******************************************************************************
-* Last Updated for Version: 6.3.7
-* Date of the Last Update:  2018-11-17
+* Last Updated for Version: 6.4.0
+* Date of the Last Update:  2019-02-12
 *
 *                    Q u a n t u m  L e a P s
 *                    ------------------------
 *                    Modern Embedded Software
 *
-* Copyright (C) 2005-2018 Quantum Leaps, LLC. All rights reserved.
+* Copyright (C) 2005-2019 Quantum Leaps. All rights reserved.
 *
 * This program is open source software: you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as published
@@ -74,8 +74,9 @@
 void QF_enterCriticalSection_(void);
 void QF_leaveCriticalSection_(void);
 
-/* set Win32 thread priority;
-* can be called either before or after QACTIVE_START().
+/* set Win32 thread priority for an active object;
+* see: Microsoft documentation for SetThreadPriority()
+* NOTE: must be called *after* QACTIVE_START()
 */
 void QF_setWin32Prio(QActive *act, int_t win32Prio);
 
@@ -103,6 +104,7 @@ int QF_consoleWaitForKey(void);
 #ifdef _MSC_VER
 
 #if (_MSC_VER < 1900) /* before Visual Studio 2015 */
+
 #define snprintf _snprintf
 #endif
 
@@ -115,7 +117,7 @@ int QF_consoleWaitForKey(void);
 #define FOPEN_S(fp_, fName_, mode_) \
     if (fopen_s(&fp_, fName_, mode_) != 0) { \
         fp_ = (FILE *)0; \
-    } else (void)0
+    } else ((void)0)
 
 #define CTIME_S(buf_, len_, time_) \
     ctime_s((char *)buf_, len_, time_)