Getting “Lock wait timeout exceeded; try restarting transaction” even though I’m not using a transaction

I’m running the following MySQL UPDATE statement:

mysql> update customer set account_import_id = 1;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

I’m not using a transaction, so why would I be getting this error? I even tried restarting my MySQL server and it didn’t help.

The table has 406,733 rows.