diff --git a/win32ss/user/ntuser/timer.c b/win32ss/user/ntuser/timer.c
index 6d4a4e983be..db029cc039d 100644
--- a/win32ss/user/ntuser/timer.c
+++ b/win32ss/user/ntuser/timer.c
@@ -183,7 +183,8 @@ IntSetTimer( PWND Window,
                   INT Type)
 {
   PTIMER pTmr;
-  UINT Ret = IDEvent;
+  UINT_PTR Ret = IDEvent;
+  ULONG ulBitmapIndex;
   LARGE_INTEGER DueTime;
   DueTime.QuadPart = (LONGLONG)(-97656); // 1024hz .9765625 ms set to 10.0 ms
 
@@ -221,22 +222,22 @@ IntSetTimer( PWND Window,
   {
       IntLockWindowlessTimerBitmap();
 
-      IDEvent = RtlFindClearBitsAndSet(&WindowLessTimersBitMap, 1, HintIndex++);
-      if (IDEvent == (UINT_PTR)-1)
+      ulBitmapIndex = RtlFindClearBitsAndSet(&WindowLessTimersBitMap, 1, HintIndex++);
+      if (ulBitmapIndex == ULONG_MAX)
       {
          HintIndex = HINTINDEX_BEGIN_VALUE;
-         IDEvent = RtlFindClearBitsAndSet(&WindowLessTimersBitMap, 1, HintIndex++);
+         ulBitmapIndex = RtlFindClearBitsAndSet(&WindowLessTimersBitMap, 1, HintIndex++);
       }
-      if (IDEvent == (UINT_PTR) -1)
+      if (ulBitmapIndex == ULONG_MAX)
       {
          IntUnlockWindowlessTimerBitmap();
          ERR("Unable to find a free window-less timer id\n");
          EngSetLastError(ERROR_NO_SYSTEM_RESOURCES);
-         ASSERT(FALSE);
          return 0;
       }
 
-      IDEvent = NUM_WINDOW_LESS_TIMERS - IDEvent;
+      ASSERT(ulBitmapIndex < NUM_WINDOW_LESS_TIMERS);
+      IDEvent = NUM_WINDOW_LESS_TIMERS - ulBitmapIndex;
       Ret = IDEvent;
 
       IntUnlockWindowlessTimerBitmap();
@@ -619,7 +620,7 @@ InitTimerImpl(VOID)
 
    RtlInitializeBitMap(&WindowLessTimersBitMap,
                        WindowLessTimersBitMapBuffer,
-                       BitmapBytes * 8);
+                       NUM_WINDOW_LESS_TIMERS);
 
    /* Yes we need this, since ExAllocatePoolWithTag isn't supposed to zero out allocated memory */
    RtlClearAllBits(&WindowLessTimersBitMap);