mysqli_real_escape_string warnings when adding fast feed

When creating a new node of type feed-fast, I get a bunch of these warnings after the import has completed:

Warning: mysqli_real_escape_string() expects parameter 2 to be a string, array given in db_escape_string() (line 410 of /var/www/html/includes/database.mysqli.inc).

The warnings seem to be harmless in that the feed items get imported correctly (as far as I can tell). There is one warning per item imported.

Running on Pressflow 6.28, PHP 5.3, mysql 5.0.95