diff --git a/src/calibre/constants.py b/src/calibre/constants.py
index cff793cd1c..e5781fe5b3 100644
--- a/src/calibre/constants.py
+++ b/src/calibre/constants.py
@@ -203,12 +203,13 @@ class Plugins(collections.Mapping):
             del sys.modules[name]
         except KeyError:
             pass
+        plugin_err = u''
         try:
-            p, err = importlib.import_module(name), ''
+            p = importlib.import_module(name)
         except Exception as err:
             p = None
-            err = str(err)
-        self._plugins[name] = (p, err)
+            plugin_err = unicode_type(err)
+        self._plugins[name] = p, plugin_err
         sys.path.remove(sys.extensions_location)
 
     def __iter__(self):