warning: mysqli_real_escape_string() expects parameter 2 to be string

Hi,

I'm running Drupal 6.20 core and I'm getting a

warning: mysqli_real_escape_string() expects parameter 2 to be string, array given in /.../includes/database.mysqli.inc on line 330.

error every time I "Update existing nodes". I have no idea what this error could be a result of because I don't know of any tools to produce a stack trace. Suggestions for how to get a stack trace on this error would be greatly appreciated (I already tried the 'Trace' module but it broke my Drupal installation).