Error message: Undefined array key “default”

Hi, I got the error message “Undefined array key “default” in file: /public_html/wp-includes/class-wp-list-util.php on line 170

The row 170 is:

$newlist[ $key ] = $value[ $field ];

In this part of the script:

/**
	 * Plucks a certain field out of each element in the input array.
	 *
	 * This has the same functionality and prototype of
	 * array_column() (PHP 5.5) but also supports objects.
	 *
	 * @since 4.7.0
	 *
	 * @param int|string $field     Field to fetch from the object or array.
	 * @param int|string $index_key Optional. Field from the element to use as keys for the new array.
	 *                              Default null.
	 * @return array Array of found values. If $index_key is set, an array of found values with keys
	 *               corresponding to $index_key. If $index_key is null, array keys from the original
	 *               $list will be preserved in the results.
	 */
	public function pluck( $field, $index_key = null ) {
		$newlist = array();

		if ( ! $index_key ) {
			/*
			 * This is simple. Could at some point wrap array_column()
			 * if we knew we had an array of arrays.
			 */
			foreach ( $this->output as $key => $value ) {
				if ( is_object( $value ) ) {
					$newlist[ $key ] = $value->$field;
				} elseif ( is_array( $value ) ) {
					$newlist[ $key ] = $value[ $field ];
				} else {
					_doing_it_wrong(
						__METHOD__,
						__( 'Values for the input array must be either objects or arrays.' ),
						'6.2.0'
					);
				}
			}

			$this->output = $newlist;

			return $this->output;
		}

		/*
		 * When index_key is not set for a particular item, push the value
		 * to the end of the stack. This is how array_column() behaves.
		 */
		foreach ( $this->output as $value ) {
			if ( is_object( $value ) ) {
				if ( isset( $value->$index_key ) ) {
					$newlist[ $value->$index_key ] = $value->$field;
				} else {
					$newlist[] = $value->$field;
				}
			} elseif ( is_array( $value ) ) {
				if ( isset( $value[ $index_key ] ) ) {
					$newlist[ $value[ $index_key ] ] = $value[ $field ];
				} else {
					$newlist[] = $value[ $field ];
				}
			} else {
				_doing_it_wrong(
					__METHOD__,
					__( 'Values for the input array must be either objects or arrays.' ),
					'6.2.0'
				);
			}
		}

		$this->output = $newlist;

		return $this->output;
	}

Do you know what the issue can be?

Thanks!

Andreas

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