diff --git a/ntoskrnl/ke/amd64/krnlinit.c b/ntoskrnl/ke/amd64/krnlinit.c
index d7aea4db738..11bc3d34a4b 100644
--- a/ntoskrnl/ke/amd64/krnlinit.c
+++ b/ntoskrnl/ke/amd64/krnlinit.c
@@ -197,7 +197,8 @@ KiSystemStartupBootStack(VOID)
 
     /* If there's no thread scheduled, put this CPU in the Idle summary */
     KiAcquirePrcbLock(Prcb);
-    if (!Prcb->NextThread) KiIdleSummary |= (ULONG_PTR)1 << Prcb->Number;
+    if (!Prcb->NextThread)
+        InterlockedBitTestAndSetAffinity(&KiIdleSummary, Prcb->Number);
     KiReleasePrcbLock(Prcb);
 
     /* Raise back to HIGH_LEVEL and clear the PRCB for the loader block */
diff --git a/ntoskrnl/ke/amd64/stubs.c b/ntoskrnl/ke/amd64/stubs.c
index 98c420ade8f..b4e6377a138 100644
--- a/ntoskrnl/ke/amd64/stubs.c
+++ b/ntoskrnl/ke/amd64/stubs.c
@@ -101,6 +101,9 @@ KiIdleLoop(VOID)
     PKPRCB Prcb = KeGetCurrentPrcb();
     PKTHREAD OldThread, NewThread;
 
+    /* Set idle summary bit */
+    InterlockedBitTestAndSetAffinity(&KiIdleSummary, Prcb->Number);
+
     /* Now loop forever */
     while (TRUE)
     {
@@ -144,9 +147,15 @@ KiIdleLoop(VOID)
             KfRaiseIrql(SYNCH_LEVEL);
 #endif
 
+            /* Clear idle summary bit */
+            InterlockedBitTestAndResetAffinity(&KiIdleSummary, Prcb->Number);
+
             /* Switch away from the idle thread */
             KiSwapContext(APC_LEVEL, OldThread);
 
+            /* Set idle summary bit */
+            InterlockedBitTestAndSetAffinity(&KiIdleSummary, Prcb->Number);
+
 #ifdef CONFIG_SMP
             /* Go back to DISPATCH_LEVEL */
             KeLowerIrql(DISPATCH_LEVEL);