diff --git a/inc/class-img-shortcode.php b/inc/class-img-shortcode.php
index 25f57ea..ac83c82 100644
--- a/inc/class-img-shortcode.php
+++ b/inc/class-img-shortcode.php
@@ -31,7 +31,7 @@ public static function get_shortcode_ui_args() {
 			$size = get_option( "${key}_size_w" );
 
 			if ( false === $size ) {
-				if ( array_key_exists( $key, $_wp_additional_image_sizes ) ) {
+				if ( isset( $_wp_additional_image_sizes ) && array_key_exists( $key, $_wp_additional_image_sizes ) ) {
 					$size = $_wp_additional_image_sizes[ $key ]['width'];
 				}
 			}