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Many functions in Drupal are not allowed to be called from inside hook_update_N(), often because they will break things when the module is updated while it is disabled.
For some fun background reading:
For Drupal 7, the suggested solution in the second issue was to document this better, so that is what this issue is about. It's pretty tricky, though. Some of your own module's API functions can be used in hook_update_N() if you specifically load your .module file, but that won't work for all of them. And a number of standard Drupal API functions won't work correctly either. See for example: