diff --git a/wp-admin/includes/class-wp-upgrader.php b/wp-admin/includes/class-wp-upgrader.php
index 80481c61c..fe2d67f0d 100644
--- a/wp-admin/includes/class-wp-upgrader.php
+++ b/wp-admin/includes/class-wp-upgrader.php
@@ -1021,6 +1021,7 @@ class Core_Upgrader extends WP_Upgrader {
 		$this->strings['downloading_package'] = __('Downloading update from <span class="code">%s</span>&#8230;');
 		$this->strings['unpack_package'] = __('Unpacking the update&#8230;');
 		$this->strings['copy_failed'] = __('Could not copy files.');
+		$this->strings['copy_failed_space'] = __('Could not copy files. You may have run out of disk space.' );
 	}
 
 	function upgrade($current) {
@@ -1073,6 +1074,9 @@ class Core_Upgrader extends WP_Upgrader {
 
 		require(ABSPATH . 'wp-admin/includes/update-core.php');
 
+		if ( ! function_exists( 'update_core' ) )
+			return new WP_Error( 'copy_failed_space', $this->strings['copy_failed_space'] );
+
 		return update_core($working_dir, $wp_dir);
 	}