diff --git a/custom_metadata.php b/custom_metadata.php
index d37c9d3..1bd8077 100644
--- a/custom_metadata.php
+++ b/custom_metadata.php
@@ -1254,7 +1254,8 @@ function _display_metadata_field( $field_slug, $field, $object_type, $object_id,
 					break;
 				case 'upload' :
 					$_attachment_id = $this->get_metadata_field_value( $field_slug . '_attachment_id', $field, $object_type, $object_id );
-					$attachment_id = array_shift( array_values( $_attachment_id ) ); // get the first value in the array
+					$_attachment_id_first_value = array_values( $_attachment_id ); //let's save it into a variable to avoid Strict Standard
+					$attachment_id = array_shift( $_attachment_id_first_value ); // get the first value in the array
 					printf( '<input type="text" name="%s" value="%s" class="custom-metadata-upload-url"%s%s/>', esc_attr( $field_id ), esc_attr( $v ), $readonly_str, $placeholder_str );
 					printf( '<input type="button" data-uploader-title="%s" data-uploader-button-text="%s" class="button custom-metadata-upload-button" value="%s"/>', esc_attr( $field->upload_modal_title ), esc_attr( $field->upload_modal_button_text ), esc_attr( $field->upload_modal_title ) );
 					printf( '<input type="button" class="button custom-metadata-clear-button" value="%s"/>', $field->upload_clear_button_text );