'DBNull' to type 'String' is not valid

Can anyone help me solve this error?  I think I am getting this because there is a NULL value in the database for the field.  Here is my code:

While objRdr.Read()

txtPhone2.Text=objRdr("SecondaryPhone")

 I also tried:

While objRdr.Read()

txtPhone2.Text=CStr(objRdr("SecondaryPhone"))

I am not using a repeater or datalist because I am only pulling a single record for the purpose of editing it.

I am still relatively new, and woulld appreciate as much detail as you can provide.  I am using SQL Server and VB.net.
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