Index: wp-admin/includes/file.php
===================================================================
--- wp-admin/includes/file.php	(revision 14373)
+++ wp-admin/includes/file.php	(working copy)
@@ -577,7 +577,7 @@
 
 	// PHP4-compat - php4 classes can't contain constants
 	$zopen = $z->open($file, /* ZIPARCHIVE::CHECKCONS */ 4);
-	if ( true !== $zopen || ZIP_ER_OK !== $zopen ) // may return true, or (int)0 ZIP_ER_OK under certain versions
+	if ( true !== $zopen || ( defined('ZIP_ER_OK') && ZIP_ER_OK !== $zopen ) ) // may return true, or (int)0 ZIP_ER_OK under certain versions
 		return new WP_Error('incompatible_archive', __('Incompatible Archive.'));
 
 	for ( $i = 0; $i < $z->numFiles; $i++ ) {