diff --git a/wp-admin/includes/file.php b/wp-admin/includes/file.php
index d8d9d59aae..cd015d91a7 100644
--- a/wp-admin/includes/file.php
+++ b/wp-admin/includes/file.php
@@ -576,7 +576,8 @@ function _unzip_file_ziparchive($file, $to, $needed_dirs = array() ) {
 	$z = new ZipArchive();
 
 	// PHP4-compat - php4 classes can't contain constants
-	if ( true !== $z->open($file, /* ZIPARCHIVE::CHECKCONS */ 4) )
+	$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
 		return new WP_Error('incompatible_archive', __('Incompatible Archive.'));
 
 	for ( $i = 0; $i < $z->numFiles; $i++ ) {