diff --git a/src/calibre/ebooks/conversion/plumber.py b/src/calibre/ebooks/conversion/plumber.py
index f5beba375d..b1d760ea2d 100644
--- a/src/calibre/ebooks/conversion/plumber.py
+++ b/src/calibre/ebooks/conversion/plumber.py
@@ -977,6 +977,8 @@ def create_oebbook(log, path_or_stream, opts, input_plugin, reader=None,
     from calibre.ebooks.oeb.base import OEBBook
     html_preprocessor = HTMLPreProcessor(input_plugin.preprocess_html,
             opts.preprocess_html, opts)
+    if not encoding:
+        encoding = None
     oeb = OEBBook(log, html_preprocessor,
             pretty_print=opts.pretty_print, input_encoding=encoding)
     if not populate:
diff --git a/src/calibre/ebooks/html/input.py b/src/calibre/ebooks/html/input.py
index 6f875ae803..1f07f4ca41 100644
--- a/src/calibre/ebooks/html/input.py
+++ b/src/calibre/ebooks/html/input.py
@@ -119,7 +119,7 @@ class HTMLFile(object):
 
         self.is_binary = level > 0 and not bool(self.HTML_PAT.search(src[:4096]))
         if not self.is_binary:
-            if encoding is None:
+            if not encoding:
                 encoding = xml_to_unicode(src[:4096], verbose=verbose)[-1]
                 self.encoding = encoding
             else:
diff --git a/src/calibre/ebooks/mobi/reader.py b/src/calibre/ebooks/mobi/reader.py
index 14e3ed11c3..57f32e7131 100644
--- a/src/calibre/ebooks/mobi/reader.py
+++ b/src/calibre/ebooks/mobi/reader.py
@@ -139,7 +139,7 @@ class BookHeader(object):
                     65001: 'utf-8',
                     }[self.codepage]
             except (IndexError, KeyError):
-                self.codec = 'cp1252' if user_encoding is None else user_encoding
+                self.codec = 'cp1252' if not user_encoding else user_encoding
                 log.warn('Unknown codepage %d. Assuming %s' % (self.codepage,
                     self.codec))
             if ident == 'TEXTREAD' or self.length < 0xE4 or 0xE8 < self.length \
diff --git a/src/calibre/gui2/convert/__init__.py b/src/calibre/gui2/convert/__init__.py
index ea7a24510a..925fecd693 100644
--- a/src/calibre/gui2/convert/__init__.py
+++ b/src/calibre/gui2/convert/__init__.py
@@ -146,6 +146,8 @@ class Widget(QWidget):
                 codecs.lookup(ans)
             except:
                 ans = ''
+            if not ans:
+                ans = None
             return ans
         elif isinstance(g, QComboBox):
             return unicode(g.currentText())