-Fixed deserializing large nullable UInt64 values

Src/Newtonsoft.Json.Tests/Serialization/JsonSerializerTest.cs

@@ -8588,6 +8588,28 @@ Path '', line 1, position 1.");
             Assert.AreEqual("derived", d.DerivedProperty);
         }
 
+        [Test]
+        public void DeserializeNullableUnsignedLong()
+        {
+            NullableLongTestClass instance = new NullableLongTestClass
+            {
+                Value = ulong.MaxValue
+            };
+            string output = JsonConvert.SerializeObject(instance);
+            NullableLongTestClass result = JsonConvert.DeserializeObject<NullableLongTestClass>(output);
+
+            Assert.AreEqual(ulong.MaxValue, result.Value);
+        }
+
+        public class NullableLongTestClass
+        {
+            public ulong? Value
+            {
+                get;
+                set;
+            }
+        }
+
 #if !NETFX_CORE
         [Test]
         public void MailMessageConverterTest()

Src/Newtonsoft.Json/Serialization/JsonSerializerInternalReader.cs

@@ -862,7 +862,7 @@ namespace Newtonsoft.Json.Serialization
 
 #if !(PORTABLE || PORTABLE40 || NET35 || NET20)
                         if (value is BigInteger)
-                            return ConvertUtils.FromBigInteger((BigInteger)value, targetType);
+                            return ConvertUtils.FromBigInteger((BigInteger)value, contract.NonNullableUnderlyingType);
 #endif
 
                         // this won't work when converting to a custom IConvertible