Dtable.Columns.Add method not adding if one column is blank

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C#3.5

Good day everybody

I am stuck with an issue i am adding values from columns into a new column called address this works fine unless there is nothing in one of the columns the it doesn't add anything at all please find my code below and an example of my data table you will see there is one that says "

Quote:

no address get created

" should not be.

Dtable.Columns.Add("Address", typeof(string), "ZoneID +','+ Suburb + ','+ PostalCode + ',' + Area").AllowDBNull = true;
            Dtable.Columns.Add("Value", typeof(string), "Suburb + '  '+ PostalCode").AllowDBNull = true;
            Select1.DataSource = Dtable;
            Select1.DataTextField = "Value";
            Select1.DataValueField = "Address";
            Select1.DataBind();

<table>
<tr>
<td>ZoneID</td>
<td>Suburb</td>
<td>Area</td>
<td>PostalCode</td>
<td>Address</td>
<td>Value</td>
</tr>
<tr>
<td>ZISA106313473401</td>
<td>PIETERSBURG</td>
<td>NULL</td>
<td>700</td>
<td>no address get created</td>
<td>PIETERSBURG  700</td>
</tr>
<tr>
<td>ZISA106313473501</td>
<td>PIETERSBURG EXT 29</td>
<td>PIETERSBURG</td>
<td>699</td>
<td>ZISA106313473501,PIETERSBURG EXT 29,699,PIETERSBURG</td>
<td>PIETERSBURG EXT 29  699</td>
</tr>
</table>

any suggestions would be appreciated