diff --git a/src/Excel/Document.php b/src/Excel/Document.php
index e46ff31..365dba3 100644
--- a/src/Excel/Document.php
+++ b/src/Excel/Document.php
@@ -76,7 +76,9 @@ class Excel_Document {
     function save($filename)
     {
         $doc = new XMLWriter();
-        $doc->openURI($filename);
+        if (!$doc->openURI($filename)) {
+            throw new Exception("unknown file " .  $filename);
+        }
         $doc->setIndent(false);
         $doc->startDocument('1.0','utf-8');
         $doc->startElement('Workbook');