Attempt to read property “slug” on null

While developing my website I frequently got warnings. When WordPress tries to access the property slug of an object that doesn’t exist (is null), it throws the following (or similar) warning:

Warning: Attempt to read property “slug” on null in taxonomy.php on line

This means somewhere in your WordPress code (specifically in taxonomy.php or a plugin/theme that called it), there’s code like: $category->slug or $term->slug but at that point, $category (or $term) is actually null — not a valid object. Here are the most likely scenarios in a WordPress context:

Invalid or deleted term ID

The code calls something like:

$term = get_term( $term_id, 'category' );
echo $term->slug;

But $term_id doesn’t correspond to a real term in the database → so get_term() returns null or a WP_Error, not an object.

Custom query or template uses an empty term

A theme or plugin loops through terms assuming all are valid, but one was deleted or is missing.

Orphaned taxonomy relationships

If your database has broken relationships (e.g., a wp_term_taxonomy entry without a valid wp_terms entry), WordPress can’t find the term and returns null.

Incorrect use of WordPress functions

This happens too. Some functions like get_the_category() or get_term_by() can return false or null if nothing is found — and code that doesn’t check that can trigger this warning.

How to fix or debug it

Add a null check before using $term->slug:

if ( ! empty( $term ) && ! is_wp_error( $term ) ) {
    echo $term->slug;
}

Verify your database integrity:

SELECT tt.term_taxonomy_id, tt.term_id
FROM wp_term_taxonomy tt
LEFT JOIN wp_terms t ON tt.term_id = t.term_id
WHERE t.term_id IS NULL;

If that returns rows, it means you have taxonomy entries that point to non-existent terms — those can cause “slug on null” warnings.

Check your custom code:

If you recently modified your theme (especially taxonomy.php, archive.php, or a custom taxonomy template), find the line mentioned in the warning and make sure the variable (e.g. $term, $category) is actually valid before accessing its properties.

Plugin conflicts:

Deactivate recently added or updated plugins that handle taxonomies — some might be mismanaging term data.

In a future post I will provide a way to detect and clean orphaned taxonomy records via an SQL query (which often cause exactly this warning).

Hope this helps.

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