Index: wp-admin/includes/ajax-actions.php
===================================================================
--- wp-admin/includes/ajax-actions.php	(revision 40198)
+++ wp-admin/includes/ajax-actions.php	(working copy)
@@ -1261,8 +1261,12 @@
 					$x->send();
 				}
 
-				if ( !$mid = add_meta( $pid ) )
+				// Check if the `add_meta` call from `edit_post` was successful (e.g. the value is not empty).
+				$mid = $wpdb->get_var( $wpdb->prepare("SELECT meta_id FROM $wpdb->postmeta WHERE post_id = %d AND meta_key = %s", $pid, wp_unslash( trim( $_POST['metakeyinput'] ) ) ) );
+
+				if ( empty( $mid ) ) {
 					wp_die( __( 'Please provide a custom field value.' ) );
+				}
 			} else {
 				wp_die( 0 );
 			}