diff --git a/src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonObject.cs b/src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonObject.cs
index 20cdc076aac64..e310382c1f3f5 100644
--- a/src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonObject.cs
+++ b/src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonObject.cs
@@ -248,17 +248,21 @@ internal void SetItem(string propertyName, JsonNode? value)
 
             OrderedDictionary<string, JsonNode?> dict = Dictionary;
 
-            if (dict.TryGetValue(propertyName, out JsonNode? replacedValue))
+            if (!dict.TryAdd(propertyName, value))
             {
+                int index = dict.IndexOf(propertyName);
+                Debug.Assert(index >= 0);
+                JsonNode? replacedValue = dict.GetAt(index).Value;
+
                 if (ReferenceEquals(value, replacedValue))
                 {
                     return;
                 }
 
                 DetachParent(replacedValue);
+                dict.SetAt(index, value);
             }
 
-            dict[propertyName] = value;
             value?.AssignParent(this);
         }