Input string was not in a correct format.

Hi,I have a login form that contains a textbox,a combobox and a password box.when all the information matches with the database it allows me to login.But i am facing the problem and it says:Input string was not in a correct format.
Here is my code:

 {

                 SqlConnection conn = new SqlConnection(dataconnection);
                 conn.Open();
                 SqlCommand cmd = new SqlCommand("uspLogin", conn);
                 cmd.Connection = conn;
                 cmd.CommandType = CommandType.StoredProcedure;
                 cmd.Parameters.AddWithValue("@ProvostName",ProvostNametextbox.Text);
           
                 cmd.Parameters.AddWithValue("@ProfessorRank",Convert.ToInt32(ProvostRankComboBox.Text));

               
                cmd.Parameters.AddWithValue("@Password", ProvostPasswordtextbox.Password);
                
                 SqlDataReader sdr = cmd.ExecuteReader();
                 if ((sdr.Read() == true))
                
                 {
                   
                     MainWindow1 main = new MainWindow1();
                     main.Welcometextblock.Text = ProvostNametextbox.Text;
                     main.Entertextblock.Text = ProvostRankComboBox.Text;
                     main.Show(); //Sending value from one form to another form.
                     Close();
                 }

                 else
                 {
                     MessageBox.Show("Sorry,try again!");

                 }

And my stored procedure is:
USE [MyHallDatabase]
GO
/****** Object:  StoredProcedure [dbo].[uspLogin]    Script Date: 01/17/2014 03:01:49 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
alter procedure [dbo].[uspLogin]
(
@ProvostName nvarchar(20),
@ProfessorRank nvarchar(20),
@Password int
)
As
begin
SELECT *
 FROM Provosts
  WHERE ProvostName=@ProvostName AND ProfessorRankId=@ProfessorRank AND Password=@Password
end


plz help me to fix it.thanks in advance.