Invalid Use Of Null

I am getting the "Invalid use of Null" message when I execute the following code under the following conditions.

fields in SQL Server table
db_date varchar(30) --> "December 29"
db_subject varchar(200) --> "Test Subject"
db_thought text --> "This is a test to see if this works"

I execute the following code from VB6

strText = "select * from db_table where db_date = 'December 29'"
ors.open strText, dbConnect
if not ors.eof then
if not isnull(ors("db_thought")) then
txtBox1 = ors("db_thought")
end if
end if

Here is my problem. Since there is a value in db_thought the code if not isnull(ors("db_thought")) evaluates to true -- this is what I expect. However when I try to assign the ors("db_thought") to the txBox1 field I get an Invalid use of Null. What am I missing?

Thanks