diff --git a/xpcom/base/nsCycleCollector.cpp b/xpcom/base/nsCycleCollector.cpp
index 63c9032f48b9..c4f349fd63bb 100644
--- a/xpcom/base/nsCycleCollector.cpp
+++ b/xpcom/base/nsCycleCollector.cpp
@@ -2077,6 +2077,11 @@ private:
   PtrInfo* AddWeakMapNode(JS::GCCellPtr aThing);
   PtrInfo* AddWeakMapNode(JSObject* aObject);
 
+  void SetFirstChild()
+  {
+    mCurrPi->SetFirstChild(mEdgeBuilder.Mark());
+  }
+
   void SetLastChild()
   {
     mCurrPi->SetLastChild(mEdgeBuilder.Mark());
@@ -2251,9 +2256,9 @@ CCGraphBuilder::BuildGraph(SliceBudget& aBudget)
 
     mCurrPi = pi;
 
-    // We need to call SetFirstChild even on deleted nodes, to set their
+    // We need to call SetFirstChild() even on deleted nodes, to set their
     // firstChild() that may be read by a prior non-deleted neighbor.
-    mCurrPi->SetFirstChild(mEdgeBuilder.Mark());
+    SetFirstChild();
 
     if (pi->mParticipant) {
       nsresult rv = pi->mParticipant->Traverse(pi->mPointer, *this);