diff --git a/lib/pdf.js b/lib/pdf.js
index 46f2ce4..300abf5 100644
--- a/lib/pdf.js
+++ b/lib/pdf.js
@@ -45,6 +45,7 @@ function PDF (html, options) {
 PDF.prototype.toBuffer = function PdfToBuffer (callback) {
   this.exec(function execPdfToBuffer (err, res) {
     if (err) return callback(err)
+    if (!res) return callback(new Error('html-pdf: PDF generation failed due to an unknown reason'))
     fs.readFile(res.filename, function readCallback (err, buffer) {
       if (err) return callback(err)
       fs.unlink(res.filename, function unlinkPdfFile (err) {