You don't pass an empty string to SQL to make it use the default values. You have to not pass the parameter at all, or maybe pass NULL to make SQL use the default value.

Maybe you can show us your SQL statement and the placeholder values you're using? Also please show us the constraints on the relevant column(s) and the default statement.

Update: See Marshall's reply below with some relevant code and some experimental evidence that a column cannot take an explicit value if you want the default to be used.