--- a/js/src/gc/Nursery.cpp
+++ b/js/src/gc/Nursery.cpp
@@ -1480,24 +1480,29 @@ void js::Nursery::shrinkAllocableSpace(s
   }
   MOZ_ASSERT(newCapacity < capacity_);
 
   unsigned newCount = JS_HOWMANY(newCapacity, ChunkSize);
   if (newCount < allocatedChunkCount()) {
     freeChunksFrom(newCount);
   }
 
+  size_t oldCapacity = capacity_;
   capacity_ = newCapacity;
   MOZ_ASSERT(capacity_ >= ArenaSize);
 
   setCurrentEnd();
 
   if (isSubChunkMode()) {
+    MOZ_ASSERT(currentChunk_ == 0);
+    chunk(0).poisonRange(newCapacity,
+                         Min(oldCapacity, NurseryChunkUsableSize) - newCapacity,
+                         JS_SWEPT_NURSERY_PATTERN, MemCheckKind::MakeNoAccess);
+
     AutoLockHelperThreadState lock;
-    MOZ_ASSERT(currentChunk_ == 0);
     decommitTask.queueRange(capacity_, chunk(0), lock);
     decommitTask.startOrRunIfIdle(lock);
   }
 }
 
 void js::Nursery::minimizeAllocableSpace() {
   shrinkAllocableSpace(tunables().gcMinNurseryBytes());
 }