diff --git a/src/libraries/System.Text.Json/Common/JsonHelpers.cs b/src/libraries/System.Text.Json/Common/JsonHelpers.cs
index 46f201cb7b5284..939e6e8fb1b0f9 100644
--- a/src/libraries/System.Text.Json/Common/JsonHelpers.cs
+++ b/src/libraries/System.Text.Json/Common/JsonHelpers.cs
@@ -133,7 +133,7 @@ public static T[] TraverseGraphWithTopologicalSort<T>(T entryNode, Func<T, IColl
                 // Iterate over the adjacency matrix, removing any occurrence of nextIndex.
                 for (int i = 0; i < adjacency.Count; i++)
                 {
-                    if (adjacency[i] is { } childMap && childMap[nextIndex])
+                    if (adjacency[i] is { } childMap && nextIndex < childMap.Length && childMap[nextIndex])
                     {
                         childMap[nextIndex] = false;