I have a logical error in my SQL retrieving function

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VB

ASP.NET

SQL-Server

I created a function to retrieve a value based on the session.

The function will take the session value and return the value I want

The problem is : it works with some rows, but it's not with others

The table I have contains more than 2000 rows

This is my function.

Public Shared Function GetUserNational(ByVal NationalID As Integer) As Int64
        Const sql = "SELECT Applicant_ID FROM DIP_Applicant where NationalID = @NationalID"
        Using con As New SqlConnection(ConfigurationManager.ConnectionStrings("mydbConnectionString").ConnectionString)
            Using cmd = New SqlCommand(sql, con)
                cmd.Parameters.AddWithValue("@NationalID", NationalID)
                con.Open()
                Using reader = cmd.ExecuteReader()
                    If reader.HasRows Then
                        reader.Read()
                        Dim count As Int64 = reader.GetInt32(0)
                        Return count
                    End If
                End Using
            End Using
        End Using
    End Function

My table contain Applicant_Id type int and primary key contain also National_Id as nvarchar(50)

in my website the user should enter his or her national Id and i will return the applicant Id

the same call the same queriy

for example I have already national_Id "102323232" when I enter it in my text box it shows me the applicant Id

but when I give anouther Id like "100555666"; it return 0 but the value should not be zero-

it's already in the table and it's not 0