Error: Blobs are not allowed in this expression

Hello everybody,
I need your help. I have one table called big _strings, and I created it using the following code:
Create table big_strings (Field_name text)
Then I wrote the following SQL Query:
Select subset (field_name, 1, 3) from big_strings;
But I get an error:
Error: Blobs are not allowed in this expression
Is there another way to select one part of a string in text field?
Thanks.
