Error, no row at position greater than 0

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VB

I am making a vb .net winform project that uses a sql server database. A user inputs the details of a person (firstName, lastName, company, contactNumber, etc.) visiting a factory into textboxes, hits the save details button, and this saves the data in the datatable. This works fine, the problem is the next part. The user is then redirected to another form where the input details are shown from the database. This works for the first record but not for any record input after that, I get an error that says "There is no row at position 'n'" and the following line of code is highlighted in the form_Load:

txtFirstName.Text = CStr(dtVisitorDetails.Rows(CInt(CDbl(txtIdNumber.Text) - 1)).Item(1))

It is telling me that any rows after row 0 are not there but I know they are because I have input them and they are showing up in the datatable in the sql server database manager.
I cannot sort this problem, any help with this would be greatly appreciated. I am attaching the rest of the code that's involved with this problem.
Thanks in advanced.

Private Sub previousVisitor_Load(sender As Object, e As EventArgs) Handles MyBase.Load

connectionString = "Data Source=.\SQLExpress;InitialCatalog=Visitors;" & _
"IntegratedSecurity=True;MultipleActiveResultSets=True"

sqlVisitorDetails = "SELECT * FROM visitorDetails WHERE idNumber=@idNumber"

    sqlCon.Open()
    sqlCmd = New SqlCommand(sqlVisitorDetails, sqlCon)
    sqlCmd.Parameters.AddWithValue("@idNumber", txtIdNumber.Text)

    dtVisitorDetails = loadDtVisitorDetails()

    txtFirstName.Text = CStr(dtVisitorDetails.Rows(CInt(CDbl(txtIdNumber.Text) - 1)).Item(1))
    txtLastName.Text = CStr(dtVisitorDetails.Rows(CInt(CDbl(txtIdNumber.Text) - 1)).Item(2))
    txtCompany.Text = CStr(dtVisitorDetails.Rows(CInt(CDbl(txtIdNumber.Text) - 1)).Item(3))
    txtContactNumber.Text = CStr(dtVisitorDetails.Rows(CInt(CDbl(txtIdNumber.Text) - 1)).Item(4))
    txtCountryCode.Text = CStr(dtVisitorDetails.Rows(CInt(CDbl(txtIdNumber.Text) - 1)).Item(5))
    txtEmail.Text = CStr(dtVisitorDetails.Rows(CInt(CDbl(txtIdNumber.Text) - 1)).Item(7))

    sqlCmd.Dispose()
    sqlCon.Close()

End Sub

Private Function loadDtVisitorDetails() As DataTable
    Dim dtVisitorDetails As DataTable = Nothing

    sqlVisitorDetails = "SELECT * FROM visitorDetails WHERE idNumber=" & txtIdNumber.Text

    dtVisitorDetails = fillDtVisitorDetails(sqlVisitorDetails)

    Return dtVisitorDetails
End Function

Public Function fillDtVisitorDetails(ByVal sqlVisitorDetails As String) As DataTable

    Dim dtVisitorDetails As New DataTable
    Dim da As New SqlDataAdapter
    Dim conCmd As New SqlCommand

    conCmd.CommandText = sqlVisitorDetails

    da.SelectCommand = conCmd
    da.SelectCommand.Connection = sqlCon

    dtVisitorDetails.Columns.GetEnumerator()
    da.Fill(dtVisitorDetails)

    Return dtVisitorDetails
End Function