diff --git a/src/calibre/ebooks/conversion/preprocess.py b/src/calibre/ebooks/conversion/preprocess.py
index 1aae939a06..33205ce4b2 100644
--- a/src/calibre/ebooks/conversion/preprocess.py
+++ b/src/calibre/ebooks/conversion/preprocess.py
@@ -238,7 +238,7 @@ class HTMLPreProcessor(object):
             )
 
         end_rules = []
-        if getattr(self.extra_opts, 'unwrap_factor', None):
+        if getattr(self.extra_opts, 'unwrap_factor', 0.0) > 0.01:
             length = line_length(html, getattr(self.extra_opts, 'unwrap_factor'))
             if length:
                 end_rules.append(
diff --git a/src/calibre/ebooks/html/input.py b/src/calibre/ebooks/html/input.py
index 008a7d104b..cf9b087295 100644
--- a/src/calibre/ebooks/html/input.py
+++ b/src/calibre/ebooks/html/input.py
@@ -262,9 +262,10 @@ class HTMLInput(InputFormatPlugin):
                 )
         ),
 
-        OptionRecommendation(name='unwrap_factor', recommended_value=None,
+        OptionRecommendation(name='unwrap_factor', recommended_value=0.0,
             help=_('Average line length for line breaking if the HTML is from a '
-                'previous partial conversion of a PDF file. Default is %default.')),
+                'previous partial conversion of a PDF file. Default is %default '
+                'which disables this.')),
 
     ])
 
diff --git a/src/calibre/gui2/convert/pdf_input.py b/src/calibre/gui2/convert/pdf_input.py
index e4a9541823..a8cacaa5d9 100644
--- a/src/calibre/gui2/convert/pdf_input.py
+++ b/src/calibre/gui2/convert/pdf_input.py
@@ -5,7 +5,7 @@ __copyright__ = '2009, John Schember <john@nachtimwald.com>'
 __docformat__ = 'restructuredtext en'
 
 from calibre.gui2.convert.pdf_input_ui import Ui_Form
-from calibre.gui2.convert import Widget
+from calibre.gui2.convert import Widget, QDoubleSpinBox
 
 class PluginWidget(Widget, Ui_Form):
 
@@ -17,3 +17,8 @@ class PluginWidget(Widget, Ui_Form):
             ['no_images', 'unwrap_factor'])
         self.db, self.book_id = db, book_id
         self.initialize_options(get_option, get_help, db, book_id)
+
+    def set_value_handler(self, g, val):
+        if val is None and isinstance(g, QDoubleSpinBox):
+            g.setValue(0.0)
+            return True