"database is locked" on first commit?

I'm having a "database is locked" problem with pysqlite.

My python script creates several threads, and creates a database 
connection in each one. The first query to run that requires a commit 
fails and produces a "database is locked" error. I'm sure no other 
queries/connections/programs/etc are modifying my database file, and 
even if they were, all modifying queries are very simple, and the 
database as a whole contains approximately 10 small records. I set the 
timeout equal to 20 seconds, so once the first execute() runs that 
contains a modifying sql statement, that thread will hang for 20 seconds 
and then produce the error I've mentioned. Subsequent execute()s using 
modifying sql also produce this error.

I searched, but I couldn't find any references to this type of problem 
online (beyond "database is locked" issues that aren't relevant here, I 
don't think).

Anyone have a suggestion or solution?

- Mike