diff --git a/src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/DataContract.cs b/src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/DataContract.cs
index 2be0f4bc9aca95..29f5bbf8e3249c 100644
--- a/src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/DataContract.cs
+++ b/src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/DataContract.cs
@@ -392,7 +392,7 @@ internal static int GetIdForInitialization(ClassDataContract classContract)
                 int currentDataContractId = DataContractCriticalHelper.s_dataContractID;
                 for (int i = 0; i < currentDataContractId; i++)
                 {
-                    if (ContractMatches(classContract, s_dataContractCache.GetItem(id)))
+                    if (ContractMatches(classContract, s_dataContractCache.GetItem(i)))
                     {
                         return i;
                     }
diff --git a/src/libraries/System.Runtime.Serialization.Xml/tests/DataContractSerializer.cs b/src/libraries/System.Runtime.Serialization.Xml/tests/DataContractSerializer.cs
index c78e842212dbdb..a03eb7db3339c7 100644
--- a/src/libraries/System.Runtime.Serialization.Xml/tests/DataContractSerializer.cs
+++ b/src/libraries/System.Runtime.Serialization.Xml/tests/DataContractSerializer.cs
@@ -1488,6 +1488,21 @@ public static void DCS_Nullables()
         Assert.Equal(item.Struct1.Value.B, actual.Struct1.Value.B);
     }
 
+    [Fact]
+    public static void DCS_NullableDataContractStructAsRoot()
+    {
+        NullableRootDataContractStruct? value = new NullableRootDataContractStruct();
+        var serializer = new DataContractSerializer(typeof(NullableRootDataContractStruct?));
+
+        using var stream = new MemoryStream();
+        serializer.WriteObject(stream, value);
+
+        stream.Position = 0;
+        var roundTripped = (NullableRootDataContractStruct?)serializer.ReadObject(stream);
+
+        Assert.True(roundTripped.HasValue);
+    }
+
     [Fact]
     public static void DCS_SimpleStructWithProperties()
     {
@@ -1545,6 +1560,11 @@ public static void DCS_PrivateTypeSerialization()
     }
 
 #region private type has to be in with in the class
+    [DataContract]
+    private struct NullableRootDataContractStruct
+    {
+    }
+
     [DataContract]
     private class PrivateType
     {