object cannot be cast from dbnull to other types in c# net

foreach (DataRow item in dt.Rows)
        {
            // while (dr1.Read())
            foreach (string s in List)
            {
                // var arr = s;

                if (s != "S-1")
                {

                    if (s == item["locationNo"])
                    {
                        st.Append("<td style="\"" mode="hold" />                                    st.Append("<td style="\"" mode="hold" />                                    st.Append("<td style="\"" mode="hold" />                                }
                    else
                    {
                        st.Append("");
                        st.Append("<tr>");
                        st.Append("<td style="\"" mode="hold" />                                    st.Append("<td style="\"" mode="hold" />                                    st.Append("<td style="\"" mode="hold" />
                    }
                }
            }
        st.Append("</tr>");
    }
}
dr.Close(); 
st.Append("");
Literal1.Text = st.ToString();