diff --git a/src/calibre/ebooks/conversion/preprocess.py b/src/calibre/ebooks/conversion/preprocess.py
index 3b1239814a..92c2fe5954 100644
--- a/src/calibre/ebooks/conversion/preprocess.py
+++ b/src/calibre/ebooks/conversion/preprocess.py
@@ -389,6 +389,7 @@ class HTMLPreProcessor(object):
             if is_pdftohtml:
                 end_rules.append((re.compile(r'<p>\s*(?P<chap>(<[ibu]>){0,2}\s*([A-Z \'"!]{3,})\s*([\dA-Z:]+\s){0,4}\s*(</[ibu]>){0,2})\s*<p>\s*(?P<title>(<[ibu]>){0,2}(\s*\w+){1,4}\s*(</[ibu]>){0,2}\s*<p>)?'), chap_head),)
 
+        length = -1
         if getattr(self.extra_opts, 'unwrap_factor', 0.0) > 0.01:
             length = line_length('pdf', html, getattr(self.extra_opts, 'unwrap_factor'))
             if length:
@@ -425,7 +426,7 @@ class HTMLPreProcessor(object):
         for rule in rules + end_rules:
             html = rule[0].sub(rule[1], html)
 
-        if is_pdftohtml:
+        if is_pdftohtml and length > -1:
             # Dehyphenate
             dehyphenator = Dehyphenator()
             html = dehyphenator(html,'pdf', length)