How do you insert into a table in a `.sdf` database?

I've tried the following:

```
string connection = @"Data Source=|DataDirectory|\InvoiceDatabase.sdf";
SqlCeConnection cn = new SqlCeConnection(connection);

try
{
   cn.Open();
}
catch (SqlCeException ex)
{
    MessageBox.Show("Connection failed");
    MessageBox.Show(ex.Message, Application.ProductName, MessageBoxButtons.OK, MessageBoxIcon.Error);
    Application.ExitThread();
}

string clientName = txt_ClientName.Text;
string address = txt_ClientAddress.Text;
string postcode = txt_postcode.Text;
string telNo = txt_TelNo.Text;

string sqlquery = ("INSERT INTO Client (Name,Address,Postcode,Telephone_Number)Values(" + clientName + "','" + address + "','" + postcode + "','" + telNo + ")");
SqlCeCommand cmd = new SqlCeCommand(sqlquery, cn);

try {
  int affectedRows = cmd.ExecuteNonQuery();

  if (affectedRows > 0)
  {
     txt_ClientAddress.Text = "";
     txt_ClientName.Text = "";
     txt_postcode.Text = "";
     txt_TelNo.Text = "";
     MessageBox.Show("Client: " + clientName + " added to database. WOoo");
  }
}
catch(Exception){
    MessageBox.Show("Insert Failed.");
} 

```

But it doesn't seem to matter what i do it just shows "Insert Failed".

Thanks in advance.