Why am I getting the error "input array is longer than the number of columns in this table"?

I have this code:

Public Function comb1(ByVal SName As String) As DataTable

        Dim dt As New DataTable
        cmd = New SqlCommand("Select Distinct RName from tb_RS_New", con)
        dr2 = cmd.ExecuteReader
        While (dr2.Read())
            dt.Rows.Add(dr2("RName"))
        End While
        Return dt

End Function

While loading the page, the error thrown was

Input array is longer than the number of columns in this table

What's wrong with my code?