--- a/xpcom/base/nsCycleCollector.cpp
+++ b/xpcom/base/nsCycleCollector.cpp
@@ -3539,16 +3539,17 @@ nsCycleCollector::FinishAnyIncrementalGC
   }
 }
 
 void
 nsCycleCollector::CleanupAfterCollection()
 {
   TimeLog timeLog;
   MOZ_ASSERT(mIncrementalPhase == CleanupPhase);
+  MOZ_RELEASE_ASSERT(!mScanInProgress);
   mGraph.Clear();
   timeLog.Checkpoint("CleanupAfterCollection::mGraph.Clear()");
 
   uint32_t interval =
     (uint32_t)((TimeStamp::Now() - mCollectionStart).ToMilliseconds());
 #ifdef COLLECT_TIME_DEBUG
   printf("cc: total cycle collector time was %ums in %u slices\n", interval,
          mResults.mNumSlices);
@@ -3779,16 +3780,17 @@ nsCycleCollector::ShouldMergeZones(ccTyp
 }
 
 void
 nsCycleCollector::BeginCollection(ccType aCCType,
                                   nsICycleCollectorListener* aManualListener)
 {
   TimeLog timeLog;
   MOZ_ASSERT(IsIdle());
+  MOZ_RELEASE_ASSERT(!mScanInProgress);
 
   mCollectionStart = TimeStamp::Now();
 
   if (mCCJSRuntime) {
     mCCJSRuntime->BeginCycleCollectionCallback();
     timeLog.Checkpoint("BeginCycleCollectionCallback()");
   }