NullException Error help.

[Jes|er]

With the code below Im getting NullException with the If statements. Where If the user enters nothing "" then the
message box shows. Any Idea's what Im doing wrong?

Private Sub btnOK_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnOK.Click

        Dim sUserID As String
        Dim sPass As String

        If sUserID.Trim() = "" Then

            MessageBox.Show("UserID cannot be blank," _
            & "Please endter a proper UserID.", "Error", _
            MessageBoxButtons.OK, MessageBoxIcon.Error)
            txtUserID.Select()
        Else
            If sPass.Trim() = "" Then
                MessageBox.Show("Password cannot be blank," _
                & "Please enter a proper password.", "Error", _
                MessageBoxButtons.OK, MessageBoxIcon.Error)
                txtPass.Select()
            Else
                Me.DialogResult = DialogResult.OK
            End If
        End If
    End Sub