CCK extraction function not found on userreference, nodereference fields when field info is cached

The first time I try to execute fieldtool_get() on a nodereference or userreference field, it works fine. Afterwards I invariably get:

Fatal error: Uncaught exception 'Exception' with message 'CCK extraction function could not be found.' in /var/www/drupal-6.14/sites/my.site.com/modules/fieldtool/plugins/fieldtool/node_cck.inc:93 Stack trace: #0 /var/www/drupal-6.14/sites/my.site.com/modules/fieldtool/fieldtool.module(256): fieldtool_node_cck_field_get(Object(stdClass), 'field_administr...', Array) #1 /var/www/drupal-6.14/sites/all/modules/devel/devel.module(1058) : eval()'d code(4): fieldtool_get(Array, Object(stdClass)) #2 /var/www/drupal-6.14/sites/all/modules/devel/devel.module(1058): eval() #3 /var/www/drupal-6.14/includes/form.inc(769): devel_execute_form_submit(Array, Array) #4 /var/www/drupal-6.14/includes/form.inc(409): form_execute_handlers('submit', Array, Array) #5 /var/www/drupal-6.14/includes/form.inc(119): drupal_process_form('devel_execute_f...', Array, Array) #6 [internal function]: drupal_get_form('devel_execute_f...') #7 /var/www/drupal-6.14/includes/menu.inc(348): call_user_func_arra in /var/www/drupal-6.14/sites/my.site.com/modules/fieldtool/plugins/fieldtool/node_cck.inc on line 93

... until I flush the fieldtool:fields cache.

Examining the cached field info with dsm() reports that before caching, 'cck extraction callback' looks like this

cck extraction callback (String, 41 characters ) fieldtool_cck_userreference_value_extract | (Callback) fieldtool_cck_userreference_value_extract();

... but when cached looks like this:

cck extraction callback (String, 41 characters ) fieldtool_cck_userreference_value_extract

Same story with 'cck itemization callback', 'transformations slot property callback', 'getter callback', 'first value getter callback', 'setter callback', and 'single value setter callback'.