diff --git src/bp-core/bp-core-filters.php src/bp-core/bp-core-filters.php
index b5e857a..09d8b42 100644
--- src/bp-core/bp-core-filters.php
+++ src/bp-core/bp-core-filters.php
@@ -848,9 +848,16 @@ function bp_setup_nav_menu_item( $menu_item ) {
 		return $menu_item;
 	}
 
+	// Prevent a notice error when using the customizer
+	$menu_classes = $menu_item->classes;
+
+	if ( is_array( $menu_classes ) ) {
+		$menu_classes = implode( ' ', $menu_item->classes);
+	}
+
 	// We use information stored in the CSS class to determine what kind of
 	// menu item this is, and how it should be treated
-	preg_match( '/\sbp-(.*)-nav/', implode( ' ', $menu_item->classes), $matches );
+	preg_match( '/\sbp-(.*)-nav/', $menu_classes, $matches );
 
 	// If this isn't a BP menu item, we can stop here
 	if ( empty( $matches[1] ) ) {
@@ -904,7 +911,11 @@ function bp_setup_nav_menu_item( $menu_item ) {
 	} else {
 		$current = bp_get_requested_url();
 		if ( strpos( $current, $menu_item->url ) !== false ) {
-			$menu_item->classes[] = 'current_page_item';
+			if ( is_array( $menu_item->classes ) ) {
+				$menu_item->classes[] = 'current_page_item';
+			} else {
+				$menu_item->classes = array( 'current_page_item' );
+			}
 		}
 	}