About capture and store in DB Text autocomplete

Hi to All,
In my form exist the field table1_field , that I have declared as Text Autocomplete and the lookup query is a Select on field table2_field.
If I select an existing value, this is stored correctly in table1_field; but if I enter manually a not existing value, the field table1_field is empty after the insert/update.
How can resolve this problem ?
Can You help me?