diff --git a/pdfminer/converter.py b/pdfminer/converter.py
index 2913ede..02545e8 100644
--- a/pdfminer/converter.py
+++ b/pdfminer/converter.py
@@ -48,7 +48,7 @@ class PDFLayoutAnalyzer(PDFTextDevice):
         return
 
     def end_page(self, page):
-        assert not self._stack, str(len(stack))
+        assert not self._stack, str(len(self._stack))
         assert isinstance(self.cur_item, LTPage), str(type(self.cur_item))
         if self.laparams is not None:
             self.cur_item.analyze(self.laparams)