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infoWordPress Core•webwordpresssystem_updateUpdatedangerousfatal_errorcodephp_errorfunctionsundefined_function•star_halfUncommon•warningModerate

Fatal error: Uncaught Error: Call to undefined function wp_is_block_theme()

The error occurs when WordPress tries to call wp_is_block_theme() but the function does not exist. This function was added in WordPress 6.8, so older installations or corrupted core files lack it.

The call often happens before core files are fully loaded, such as in a mu‑plugin, theme functions.php, or an early hook. When the function is missing, PHP throws a fatal error and stops execution.

Symptoms

[dashicons-warning]
White screen of death
Site shows a blank page or technical difficulties message
[dashicons-warning]
Fatal error in browser
Message displays: Fatal error: Uncaught Error: Call to undefined function wp_is_block_theme()
[dashicons-admin-generic]
Admin area fails to load
Customizer, plugins page, or theme editor is unable to open

Common Causes

Outdated WordPress version
Site runs a version earlier than 6.8 where the function does not exist
Corrupted core files
Missing or truncated wp-includes/theme.php prevents the function from being defined
Early call to function
Plugin, theme, or mu‑plugin invokes wp_is_block_theme() before core is ready
Server restrictions
File permissions or open_basedir block access to wp-includes directory
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