diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index c09f70423d..92b811830c 100644
--- a/src/runtime/mgc.go
+++ b/src/runtime/mgc.go
@@ -1121,6 +1121,10 @@ top:
 		// finalizers have been scanned.
 		work.finalizersDone = true
 
+		// Disable assists and background workers. We must do
+		// this before waking blocked assists.
+		atomic.Store(&gcBlackenEnabled, 0)
+
 		// Flush the gcWork caches. This must be done before
 		// endCycle since endCycle depends on statistics kept
 		// in these caches.
diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go
index 52545afa29..eac45ec168 100644
--- a/src/runtime/mgcmark.go
+++ b/src/runtime/mgcmark.go
@@ -491,7 +491,8 @@ retry:
 }
 
 // gcWakeAllAssists wakes all currently blocked assists. This is used
-// at the end of a GC cycle.
+// at the end of a GC cycle. gcBlackenEnabled must be false to prevent
+// new assists from going to sleep after this point.
 func gcWakeAllAssists() {
 	lock(&work.assistQueue.lock)
 	injectglist(work.assistQueue.head.ptr())