diff --git a/admin-functions.php b/admin-functions.php
index f3df87c0d..a51d65dd7 100644
--- a/admin-functions.php
+++ b/admin-functions.php
@@ -151,10 +151,13 @@ function duplicate_post_plugin_upgrade() {
 	);
 	add_option( 'duplicate_post_show_link_in', $show_links_in_defaults );
 
-	$taxonomies_blacklist = get_option( 'duplicate_post_taxonomies_blacklist' );
-	if ( $taxonomies_blacklist === '' ) {
+	$taxonomies_blacklist = get_option( 'duplicate_post_taxonomies_blacklist', [] );
+	if ( empty( $taxonomies_blacklist ) ) {
 		$taxonomies_blacklist = [];
 	}
+	elseif ( ! is_array( $taxonomies_blacklist ) ) {
+		$taxonomies_blacklist = [ $taxonomies_blacklist ];
+	}
 	if ( in_array( 'post_format', $taxonomies_blacklist, true ) ) {
 		update_option( 'duplicate_post_copyformat', 0 );
 		$taxonomies_blacklist = array_diff( $taxonomies_blacklist, [ 'post_format' ] );
@@ -293,10 +296,13 @@ function duplicate_post_copy_post_taxonomies( $new_id, $post ) {
 			$post_taxonomies[] = 'post_format';
 		}
 
-		$taxonomies_blacklist = get_option( 'duplicate_post_taxonomies_blacklist' );
-		if ( $taxonomies_blacklist === '' ) {
+		$taxonomies_blacklist = get_option( 'duplicate_post_taxonomies_blacklist', [] );
+		if ( empty( $taxonomies_blacklist ) ) {
 			$taxonomies_blacklist = [];
 		}
+		elseif ( ! is_array( $taxonomies_blacklist ) ) {
+			$taxonomies_blacklist = [ $taxonomies_blacklist ];
+		}
 		if ( intval( get_option( 'duplicate_post_copyformat' ) ) === 0 ) {
 			$taxonomies_blacklist[] = 'post_format';
 		}