Type TypePoint Is Marked For Native Serialization, But Field X Of Type TypePoint Is Not Valid For Native Serialization

I made a point type by C# for deploying in SQL2005.

It builds successfully every time.

When I deploy, it gives the following error:
"Error: Type "SqlServerProject1.TypePoint" is marked for native serialization, but field "x" of type "SqlServerProject1.TypePoint" is not valid for native serialization." although x variable is value type decimal.

using System;
using System.Data;
using System.Data.SqlClient;
using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;

[Serializable]
[SqlUserDefinedType(Format.Native, IsByteOrdered=true)]
public struct TypePoint : INullable
{
private bool m_Null;

private Decimal x;
private Decimal y;

public override string ToString()
{
// Replace the following code with your code

if (IsNull) // added part
{
return "null";
}
else
{
return x + ":" + y;
} // end of this part

// return "";
}

public bool IsNull
{
get
{
// Put your code here
return m_Null;
}
}

public static TypePoint Null
{
get
{
TypePoint h = new TypePoint();
h.m_Null = true;
return h;
}
}

public static TypePoint Parse(SqlString s) // Most Important Part
{
if (s.IsNull)
{
return Null;
}
else // added part
{
TypePoint p = new TypePoint();
string str = s.ToString();
string[] xy = str.Split(':');
p.x = Decimal.Parse(xy[0]);
p.y = Decimal.Parse(xy[1]);
return p;
}
}

public Decimal X
{
get
{
return x;
}
set
{
x = value;
}
}

public Decimal Y
{
get
{
return y;
}
set
{
y = value;
}
} // end of added part

// This is a place-holder method
public string Method1()
{
//Insert method code here
return "Hello";
}

// This is a place-holder static method
public static SqlString Method2()
{
//Insert method code here
return new SqlString("Hello");
}

// This is a place-holder field member
public int var1;
}