This bug manifests itself with the pysqlite backend
with a property of type Boolean.

I think the problem is with the test to convert the value 
to SQL format. The test is under the "if value:" condition,
so the conversion is not done if the value is false. 
In such case, the value 'False' goes to the backend
and the pysqlite backend does not like that.

The attached patch seems to fix the problem.

-sbi