diff --git a/docshell/shistory/SessionHistoryEntry.cpp b/docshell/shistory/SessionHistoryEntry.cpp
index 90d9199d8dfe..5325205f406f 100644
--- a/docshell/shistory/SessionHistoryEntry.cpp
+++ b/docshell/shistory/SessionHistoryEntry.cpp
@@ -805,6 +805,8 @@ void SessionHistoryEntry::AddChild(SessionHistoryEntry* aChild, int32_t aOffset,
           "Adding a child where we already have a child? This may misbehave");
       oldChild->SetParent(nullptr);
     }
+  } else {
+    mChildren.SetLength(aOffset + 1);
   }
 
   mChildren.ReplaceElementAt(aOffset, aChild);