diff --git a/src/calibre/ebooks/txt/input.py b/src/calibre/ebooks/txt/input.py
index 86d5e23d10..f6900a5a2a 100644
--- a/src/calibre/ebooks/txt/input.py
+++ b/src/calibre/ebooks/txt/input.py
@@ -10,7 +10,7 @@ from calibre.customize.conversion import InputFormatPlugin
 from calibre.ebooks.txt.processor import txt_to_markdown, opf_writer
 
 class TXTInput(InputFormatPlugin):
-    
+
     name        = 'TXT Input'
     author      = 'John Schember'
     description = 'Convert TXT files to HTML'
@@ -22,16 +22,20 @@ class TXTInput(InputFormatPlugin):
         if options.input_encoding:
             ienc = options.input_encoding
         txt = stream.read().decode(ienc)
-        
-        html = txt_to_markdown(txt)
+
+        try:
+            html = txt_to_markdown(txt)
+        except RuntimeError:
+            raise ValueError('This txt file has malformed markup, it cannot be'
+                'converted by calibre. See http://daringfireball.net/projects/markdown/syntax')
         with open('index.html', 'wb') as index:
             index.write(html.encode('utf-8'))
-            
+
         from calibre.ebooks.metadata.meta import get_metadata
         mi = get_metadata(stream, 'txt')
         manifest = [('index.html', None)]
         spine = ['index.html']
         opf_writer(os.getcwd(), 'metadata.opf', manifest, spine, mi)
-        
+
         return os.path.join(os.getcwd(), 'metadata.opf')